diff --git a/static/js/components/resourceIcon.js b/static/js/components/resourceIcon.js
index ec0a797d..6f50d790 100644
--- a/static/js/components/resourceIcon.js
+++ b/static/js/components/resourceIcon.js
@@ -36,6 +36,13 @@ function buildResourceIcon(item, resourceType) {
 
     const canThumbnail = thumbnail?.canHandle(file) ?? false;
     if (canThumbnail) {
+        // A PDF just entered the list: warm up the pdf.js stack (~1.3 MB)
+        // in the background now, so a thumbnail cache-miss below doesn't
+        // stall its first render on the library download. Idempotent.
+        if (file.mime_type === 'application/pdf') {
+            thumbnail.preloadPdf();
+        }
+
         const img = document.createElement('img');
         img.className = 'file-thumb';
         img.src = `/api/files/${file.id}/thumbnail/icon`;
diff --git a/static/js/components/resourceList.js b/static/js/components/resourceList.js
index 73b633fc..63c82380 100644
--- a/static/js/components/resourceList.js
+++ b/static/js/components/resourceList.js
@@ -140,6 +140,18 @@ export class ResourceListComponent {
          */
         this._lastGroupEl = null;
 
+        /**
+         * Live swimlane wrappers currently in the DOM, keyed by group key —
+         * lets `_findLaneByKey()` resolve in O(1) instead of a container-wide
+         * `querySelector` per lookup. Kept in sync with the DOM: entries are
+         * added where lanes are created (`_appendItems`,
+         * `_ensureJustAddedLane`) and the map is cleared on the full-container
+         * wipes in `render()` / `clear()`; lanes are never removed
+         * individually anywhere else.
+         * @type {Map<string, HTMLElement>}
+         */
+        this._lanes = new Map();
+
         /**
          * Optional grouping-key resolver stored between `render()` / `append()`
          * calls so `addItem()` can place a new row in the correct swimlane
@@ -203,6 +215,7 @@ export class ResourceListComponent {
         // Reset group tracking for the fresh render
         this._lastGroupKey = undefined;
         this._lastGroupEl = null;
+        this._lanes.clear();
         this._groupFn = groupFn;
         this._groupLabelFn = groupLabelFn;
         this._headerNodeFn = headerNodeFn;
@@ -244,6 +257,7 @@ export class ResourceListComponent {
         this._lastClickedIndex = -1;
         this._lastGroupKey = undefined;
         this._lastGroupEl = null;
+        this._lanes.clear();
         this._groupFn = undefined;
         this._groupLabelFn = undefined;
         this._headerNodeFn = undefined;
@@ -348,7 +362,8 @@ export class ResourceListComponent {
             // "New" swimlane, creating it on first call.
             const lane = this._ensureJustAddedLane();
             this._items.set(item.id, item);
-            row = isFile ? this._createFileItem(/** @type {FileItem} */ (item)) : this._createFolderItem(/** @type {FolderItem} */ (item));
+            const labels = this._buildItemLabels();
+            row = isFile ? this._createFileItem(/** @type {FileItem} */ (item), labels) : this._createFolderItem(/** @type {FolderItem} */ (item), labels);
             lane.appendChild(row);
         } else {
             // Flat list (no grouping) — append at the end like before.
@@ -394,6 +409,7 @@ export class ResourceListComponent {
         const lane = document.createElement('div');
         lane.className = 'resource-list__swimlane-group resource-list__swimlane-group--just-added';
         lane.dataset.groupKey = JUST_ADDED_KEY;
+        this._lanes.set(JUST_ADDED_KEY, lane);
 
         const header = document.createElement('div');
         header.className = 'resource-list__swimlane-header';
@@ -420,13 +436,14 @@ export class ResourceListComponent {
      * Locate an on-screen swimlane wrapper by its group key. Returns
      * `null` when no swimlane currently matches.
      *
+     * O(1) via the `_lanes` registry — see its declaration for how it is
+     * kept in sync with the DOM.
+     *
      * @param {string} key
      * @returns {HTMLElement | null}
      */
     _findLaneByKey(key) {
-        // CSS.escape covers arbitrary key shapes (dates with colons,
-        // UUIDs with dashes, etc.) so the attribute selector is safe.
-        return /** @type {HTMLElement | null} */ (this._container.querySelector(`.resource-list__swimlane-group[data-group-key="${CSS.escape(String(key))}"]`));
+        return this._lanes.get(String(key)) ?? null;
     }
 
     /**
@@ -523,6 +540,9 @@ export class ResourceListComponent {
     _appendItems(items, groupFn, groupLabelFn, headerNodeFn) {
         const fragment = document.createDocumentFragment();
 
+        // Resolve batch-invariant labels once, not once per row.
+        const labels = this._buildItemLabels();
+
         // Start from the persisted key so load-more pages continue seamlessly.
         let lastGroupKey = this._lastGroupKey;
 
@@ -545,11 +565,12 @@ export class ResourceListComponent {
                     if (key !== null) {
                         fragmentGroup = document.createElement('div');
                         fragmentGroup.className = 'resource-list__swimlane-group';
-                        // Stamp the group key on the wrapper so `addItem()`
-                        // can locate this swimlane later via
-                        // `_findLaneByKey()` and append into it without a
-                        // full re-render.
+                        // Stamp the group key on the wrapper (handy in
+                        // devtools) and register it in `_lanes` so
+                        // `_findLaneByKey()` can locate this swimlane later
+                        // without a container-wide query.
                         fragmentGroup.dataset.groupKey = key;
+                        this._lanes.set(key, fragmentGroup);
                         fragmentGroup.appendChild(this._createGroupHeader(key, groupLabelFn, headerNodeFn));
                         fragment.appendChild(fragmentGroup);
                     }
@@ -558,7 +579,9 @@ export class ResourceListComponent {
 
             // Dispatch to the correct renderer: files have mime_type, folders do not.
             const isFile = 'mime_type' in item;
-            const itemEl = isFile ? this._createFileItem(/** @type {FileItem} */ (item)) : this._createFolderItem(/** @type {FolderItem} */ (item));
+            const itemEl = isFile
+                ? this._createFileItem(/** @type {FileItem} */ (item), labels)
+                : this._createFolderItem(/** @type {FolderItem} */ (item), labels);
 
             // Priority: live DOM group (load-more continuation) > current fragment group > bare container
             const target = liveGroup ?? fragmentGroup;
@@ -600,12 +623,50 @@ export class ResourceListComponent {
         return el;
     }
 
+    /**
+     * @typedef {Object} ItemLabels
+     * @property {string} folderTypeLabel - Type-cell label for folders.
+     * @property {string} customActionsHtml - Pre-rendered inline-action buttons.
+     * @property {(category: string) => string} fileTypeLabel - Type-cell label
+     *   for a file category (memoized per batch).
+     */
+
+    /**
+     * Resolve every per-row value that does not depend on the item once per
+     * batch: the i18n lookups for the type cell and the custom-actions HTML
+     * are identical for all 50 rows of a page, so repeating them in
+     * `_createFileItem` / `_createFolderItem` was pure overhead. Built fresh
+     * on every call (never cached on the instance), so a locale switch is
+     * picked up naturally by the next render/append.
+     *
+     * @returns {ItemLabels}
+     */
+    _buildItemLabels() {
+        const fallbackTypeLabel = i18n.t('files.file_types.document');
+        /** @type {Map<string, string>} */
+        const byCategory = new Map();
+        return {
+            folderTypeLabel: i18n.t('files.file_types.folder'),
+            customActionsHtml: this._renderCustomActions(),
+            fileTypeLabel(category) {
+                if (!category) return fallbackTypeLabel;
+                let label = byCategory.get(category);
+                if (label === undefined) {
+                    label = i18n.t(`files.file_types.${category.toLowerCase()}`) || category;
+                    byCategory.set(category, label);
+                }
+                return label;
+            }
+        };
+    }
+
     /**
      * Build a .file-item DOM element for a folder.
      * @param {FolderItem} folder
+     * @param {ItemLabels} labels - Batch-invariant labels from `_buildItemLabels()`.
      * @returns {HTMLElement}
      */
-    _createFolderItem(folder) {
+    _createFolderItem(folder, labels) {
         const cfg = this._cfg;
         const el = document.createElement('div');
         const modClass = cfg.itemModifierClass ? ` ${cfg.itemModifierClass}` : '';
@@ -632,11 +693,11 @@ export class ResourceListComponent {
             </div>
             <div class="owner-cell${this._ownerVisible ? '' : ' hidden'}" data-owner-id="${escapeHtml(folder.owner_id || '')}"></div>
             ${cfg.showPath ? `<div class="path-cell" title="${escapeHtml(folder.path || '')}">${escapeHtml(folder.path || '')}</div>` : ''}
-            ${cfg.showType ? `<div class="type-cell">${i18n.t('files.file_types.folder')}</div>` : ''}
+            ${cfg.showType ? `<div class="type-cell">${labels.folderTypeLabel}</div>` : ''}
             <div class="size-cell">--</div>
             <div class="date-cell">${formattedDate}</div>
             <div class="action-cell">
-                ${this._renderCustomActions()}
+                ${labels.customActionsHtml}
                 ${cfg.showFavorite ? `<button class="favorite-star${isFav ? ' active' : ''}"><i class="${isFav ? 'fas' : 'far'} fa-star"></i></button>` : ''}
                 ${cfg.showContextMenu ? '<button class="file-actions"><i class="fas fa-ellipsis-v"></i></button>' : ''}
             </div>
@@ -649,12 +710,12 @@ export class ResourceListComponent {
     /**
      * Build a .file-item DOM element for a file.
      * @param {FileItem} file
+     * @param {ItemLabels} labels - Batch-invariant labels from `_buildItemLabels()`.
      * @returns {HTMLElement}
      */
-    _createFileItem(file) {
+    _createFileItem(file, labels) {
         const cfg = this._cfg;
-        const cat = file.category || '';
-        const typeLabel = cat ? i18n.t(`files.file_types.${cat.toLowerCase()}`) || cat : i18n.t('files.file_types.document');
+        const typeLabel = labels.fileTypeLabel(file.category || '');
         const fileSize = file.size_formatted || formatFileSize(file.size);
         const dateVal = /** @type {Record<string,string>} */ (/** @type {unknown} */ (file))[cfg.dateField] ?? file.modified_at;
         const formattedDate = cfg.dateFormatter ? cfg.dateFormatter(dateVal) : formatDateTime(new Date(dateVal));
@@ -685,7 +746,7 @@ export class ResourceListComponent {
             <div class="size-cell">${fileSize}</div>
             <div class="date-cell">${formattedDate}</div>
             <div class="action-cell">
-                ${this._renderCustomActions()}
+                ${labels.customActionsHtml}
                 ${cfg.showFavorite ? `<button class="favorite-star${isFav ? ' active' : ''}"><i class="${isFav ? 'fas' : 'far'} fa-star"></i></button>` : ''}
                 ${cfg.showContextMenu ? '<button class="file-actions"><i class="fas fa-ellipsis-v"></i></button>' : ''}
             </div>
diff --git a/static/js/features/thumbnail.js b/static/js/features/thumbnail.js
index 17f23922..c0aa0ea3 100644
--- a/static/js/features/thumbnail.js
+++ b/static/js/features/thumbnail.js
@@ -2,29 +2,47 @@ import { getCsrfHeaders } from '../core/csrf.js';
 
 /** @import {FileItem} from '../core/types.js' */
 
+// IMPORTANT: absolute paths so the dynamic import resolves correctly both in
+// dev mode (native ESM, module at /js/features/thumbnail.js) and in release
+// mode (IIFE bundle at /js/app.{hash}.js — relative '../vendors/…' would
+// incorrectly resolve to /vendors/… instead of /js/vendors/…).
+const PDFJS_LIB_URL = '/js/vendors/pdf.min.mjs';
+const PDFJS_WORKER_URL = '/js/vendors/pdf.worker.min.mjs';
+
 /**
- * use any type so tsc will not scan library
- * @type {any}
+ * Memoized import of pdf.min.mjs (in-flight or settled).
+ * use any type so tsc will not scan library.
+ * Reset to null on failure so a later call retries (e.g. transient offline).
+ * @type {Promise<any> | null}
  */
-let _pdfjsLib = null;
+let _pdfjsLibPromise = null;
+
+/** True once the worker script warm-up fetch has completed successfully. */
+let _pdfWorkerWarmed = false;
 
 // TODO: do we need to add a max concurrncy ?
 
 /**
  * Lazy-loads pdf.min.mjs on first use via dynamic import so it is never
  * bundled into the IIFE (it uses top-level await which breaks IIFE wrapping).
+ * Memoizing the promise (rather than the resolved module) lets concurrent
+ * callers — e.g. `preloadPdf()` racing the first real thumbnail — share a
+ * single network fetch.
  * @returns {Promise<any>}
  */
-async function getPdfjsLib() {
-    if (_pdfjsLib) return _pdfjsLib;
-    // IMPORTANT: use an absolute path so the import resolves correctly both in
-    // dev mode (native ESM, module at /js/features/thumbnail.js) and in release
-    // mode (IIFE bundle at /js/app.{hash}.js — relative '../vendors/…' would
-    // incorrectly resolve to /vendors/… instead of /js/vendors/…).
-    const lib = '/js/vendors/pdf.min.mjs';
-    _pdfjsLib = /** @type {any} */ (await import(lib));
-    _pdfjsLib.GlobalWorkerOptions.workerSrc = '/js/vendors/pdf.worker.min.mjs';
-    return _pdfjsLib;
+function getPdfjsLib() {
+    if (!_pdfjsLibPromise) {
+        _pdfjsLibPromise = import(PDFJS_LIB_URL)
+            .then((lib) => {
+                lib.GlobalWorkerOptions.workerSrc = PDFJS_WORKER_URL;
+                return lib;
+            })
+            .catch((err) => {
+                _pdfjsLibPromise = null; // allow retry after a failed load
+                throw err;
+            });
+    }
+    return _pdfjsLibPromise;
 }
 
 export const thumbnail = {
@@ -43,6 +61,33 @@ export const thumbnail = {
         return false;
     },
 
+    /**
+     * Fire-and-forget warm-up of the pdf.js stack (module + worker script).
+     *
+     * Called the moment a PDF row enters the DOM (see resourceIcon.js), so
+     * the ~1.3 MB library downloads in the background while the user is
+     * still looking at the list — instead of stalling the first thumbnail
+     * render on it. Idempotent and cheap after the first call, and only
+     * folders that actually contain PDFs ever pay the download.
+     */
+    preloadPdf() {
+        // Module (≈300 KB): shares the memoized promise with real users.
+        getPdfjsLib().catch(() => {
+            /* transient failure — the first real use retries */
+        });
+
+        // Worker (≈1 MB): pdf.js only fetches it via `new Worker(...)` on the
+        // first getDocument(), so prime the HTTP cache with a plain fetch.
+        // Reading the body ensures the download completes and is cacheable.
+        if (_pdfWorkerWarmed) return;
+        _pdfWorkerWarmed = true;
+        fetch(PDFJS_WORKER_URL)
+            .then((r) => (r.ok ? r.blob() : Promise.reject(new Error(`HTTP ${r.status}`))))
+            .catch(() => {
+                _pdfWorkerWarmed = false; // allow retry on a later sighting
+            });
+    },
+
     // TODO: use these informations from server ?
     SIZES: {
         icon: { width: 150, height: 150 },
diff --git a/static/js/views/admin/admin.js b/static/js/views/admin/admin.js
index 6bcd5843..6a3330d6 100644
--- a/static/js/views/admin/admin.js
+++ b/static/js/views/admin/admin.js
@@ -151,6 +151,10 @@ function switchTab(name, el) {
     }
 
     activeTabName = name;
+    // The migration auto-poll only makes sense while the Storage tab is
+    // visible — without this it would keep hitting the API every 2 s
+    // (and updating hidden DOM) for as long as a migration runs.
+    if (name !== 'storage') stopMigrationPolling();
     if (name === 'users') loadUsers();
     if (name === 'dashboard') loadDashboard();
     if (name === 'storage') loadStorage();
@@ -908,6 +912,22 @@ async function testStorageConnection() {
 /** @type {ReturnType<typeof setInterval> | null} */
 let migrationPollTimer = null;
 
+/**
+ * Stop the 2 s migration auto-poll if it is armed.
+ *
+ * Called when the poll observes a non-running status, when a poll request
+ * fails (an expired admin session would otherwise be retried every 2 s
+ * forever), and when the user leaves the Storage tab. Re-entering the tab
+ * re-arms it via `loadStorage()` → `loadMigrationStatus()` while a
+ * migration is running.
+ */
+function stopMigrationPolling() {
+    if (migrationPollTimer) {
+        clearInterval(migrationPollTimer);
+        migrationPollTimer = null;
+    }
+}
+
 /**
  * @param {string} msg
  * @param {string} type
@@ -979,7 +999,12 @@ async function loadMigrationStatus() {
             headers: headers(),
             credentials: 'same-origin'
         });
-        if (!resp.ok) return;
+        if (!resp.ok) {
+            // Don't keep hammering a failing endpoint (e.g. expired session);
+            // any migration button or tab re-entry re-arms the poll.
+            stopMigrationPolling();
+            return;
+        }
         const m = await resp.json();
         updateMigrationUI(m);
 
@@ -988,9 +1013,8 @@ async function loadMigrationStatus() {
             if (!migrationPollTimer) {
                 migrationPollTimer = setInterval(loadMigrationStatus, 2000);
             }
-        } else if (migrationPollTimer) {
-            clearInterval(migrationPollTimer);
-            migrationPollTimer = null;
+        } else {
+            stopMigrationPolling();
         }
     } catch (_e) {
         /* ignore */