diff --git a/static/js/app.js b/static/js/app.js
index b5d9891..5465c34 100644
--- a/static/js/app.js
+++ b/static/js/app.js
@@ -1815,11 +1815,21 @@ async function parseEpub(arrayBuffer) {
     .map(ref => manifest[ref.getAttribute('idref')]?.href)
     .filter(Boolean);
 
+  // All chapters unpacked at once: one after the other, every chapter waits for
+  // JSZip's own ticks (a book like Das Kapital has 148 of them).
+  const texts = await Promise.all(spineItems.map(href => {
+    const f = zip.file(href);
+    return f ? f.async('text').catch(() => null) : null;
+  }));
+
   const chapters = [];
+  let slice = performance.now();
   for (let i = 0; i < spineItems.length; i++) {
     const href = spineItems[i];
+    if (texts[i] == null) continue;   // missing
     try {
-      let chapterText = await zip.file(href).async('text');
+      let chapterText = texts[i];
+      texts[i] = null;
       // Strip style/script blocks early — publisher EPUBs often embed large base64 fonts in CSS.
       // This shrinks the string before regex and DOMParser work on it.
       chapterText = chapterText.replace(/<style\b[^>]*>[\s\S]*?<\/style>/gi, '');
@@ -1828,8 +1838,12 @@ async function parseEpub(arrayBuffer) {
       const withBlobs = _injectImageBlobs(chapterText, chapterDir, imageMap);
       const sanitized = sanitizeEpubHtml(withBlobs);
       chapters.push(`<div id="epub-chapter-${i}" data-epub-src="${href}">${sanitized}</div>`);
-      if (i % 8 === 0) await new Promise(r => requestAnimationFrame(r));
-    } catch (e) { /* skip missing */ }
+      // Let the page breathe (spinner, taps) every ~50 ms of work, not after a fixed count.
+      if (performance.now() - slice > 50) {
+        await new Promise(r => setTimeout(r, 0));
+        slice = performance.now();
+      }
+    } catch (e) { /* skip unreadable */ }
   }
 
   return {title, author, chapters, toc, imageMap};
@@ -2222,11 +2236,12 @@ function _cmpProgress(anchorA, fracA, anchorB, fracB) {
 const _BOOK_CACHE_STORE       = 'books';
 const _BOOK_META_STORE        = 'book_meta';
 const _BOOK_ANNOTATIONS_STORE = 'book_annotations';
+const _BOOK_PARSED_STORE      = 'parsed';   // prepared chapters, see _getParsedBook
 const _BOOK_CACHE_EVICT_MS    = 4 * 7 * 24 * 60 * 60 * 1000;
 
 function _openBookCacheDb() {
   return new Promise((resolve, reject) => {
-    const req = indexedDB.open('diora_book_cache', 2);
+    const req = indexedDB.open('diora_book_cache', 3);
     req.onupgradeneeded = e => {
       const db = e.target.result;
       if (!db.objectStoreNames.contains(_BOOK_CACHE_STORE))
@@ -2235,12 +2250,98 @@ function _openBookCacheDb() {
         db.createObjectStore(_BOOK_META_STORE, {keyPath: 'id'});
       if (!db.objectStoreNames.contains(_BOOK_ANNOTATIONS_STORE))
         db.createObjectStore(_BOOK_ANNOTATIONS_STORE, {keyPath: 'id'});
+      if (!db.objectStoreNames.contains(_BOOK_PARSED_STORE))
+        db.createObjectStore(_BOOK_PARSED_STORE, {keyPath: 'id'});
     };
     req.onsuccess  = e => resolve(e.target.result);
     req.onerror    = ()  => reject();
   });
 }
 
+// ---------------------------------------------------------------------------
+// Prepared books: an EPUB's chapters after unpacking and sanitizing, so opening
+// it again skips both (for a book like Das Kapital that's most of the time an
+// e-reader needs to open it). Encrypted with the book key like everything else
+// on the device. Valid for one version of the book file (its IV) and of the
+// code that prepares chapters (a hash of its source: changing the sanitizer
+// makes every book be prepared anew). Images stay in the book file: the
+// chapters keep "diora-img:<path>" in place of the session's blob URLs.
+// ---------------------------------------------------------------------------
+let _parseCodeSig = null;
+
+function _parsePreparationSig() {
+  if (_parseCodeSig) return _parseCodeSig;
+  const src = [parseEpub, sanitizeEpubHtml, _injectImageBlobs, _resolveImageBlob, _parseEpubToc]
+    .map(f => String(f)).join('\n');
+  let h = 5381;
+  for (let i = 0; i < src.length; i++) h = ((h << 5) + h + src.charCodeAt(i)) | 0;
+  return (_parseCodeSig = (h >>> 0).toString(36) + '.' + src.length);
+}
+
+async function _getParsedBook(bookId, dataIv, key) {
+  try {
+    const db = await _openBookCacheDb();
+    const row = await new Promise(resolve => {
+      const req = db.transaction(_BOOK_PARSED_STORE).objectStore(_BOOK_PARSED_STORE).get(bookId);
+      req.onsuccess = e => resolve(e.target.result || null);
+      req.onerror = () => resolve(null);
+    });
+    if (!row || row.sig !== dataIv + ':' + _parsePreparationSig()) return null;
+    const plain = await crypto.subtle.decrypt({name: 'AES-GCM', iv: row.iv}, key, row.ct);
+    return JSON.parse(new TextDecoder().decode(plain));
+  } catch (e) { return null; }
+}
+
+async function _setParsedBook(bookId, dataIv, key, result) {
+  try {
+    // Blob URLs live as long as this page; the stored chapters name the image instead.
+    const pathOf = {};
+    for (const [path, url] of Object.entries(result.imageMap || {})) pathOf[url] = path;
+    const blobRe = /blob:[^"'\s)]+/g;
+    const chapters = result.chapters.map(c => c.replace(blobRe, u => pathOf[u] ? 'diora-img:' + pathOf[u] : u));
+    const doc = {title: result.title, author: result.author, toc: result.toc, chapters,
+                 images: Object.keys(result.imageMap || {})};
+    // Raw bytes, not base64: IndexedDB keeps an ArrayBuffer as it is (megabytes for a big book).
+    const iv = crypto.getRandomValues(new Uint8Array(12));
+    const ct = await crypto.subtle.encrypt({name: 'AES-GCM', iv}, key, new TextEncoder().encode(JSON.stringify(doc)));
+    const db = await _openBookCacheDb();
+    await new Promise(resolve => {
+      const tx = db.transaction(_BOOK_PARSED_STORE, 'readwrite');
+      const store = tx.objectStore(_BOOK_PARSED_STORE);
+      store.put({id: bookId, sig: dataIv + ':' + _parsePreparationSig(), iv, ct, cached_at: Date.now()});
+      // Prepared books not opened for a while go (they come back on the next open).
+      const all = store.getAll();
+      all.onsuccess = () => all.result.forEach(r => {
+        if (r.id !== bookId && Date.now() - (r.cached_at || 0) > _BOOK_CACHE_EVICT_MS) store.delete(r.id);
+      });
+      tx.oncomplete = resolve;
+      tx.onerror = resolve;
+    });
+  } catch (e) {}
+}
+
+/** A prepared book back to what parseEpub returns: fresh blob URLs for its images. */
+async function _restoreParsedBook(doc, epubBytes) {
+  const imageMap = {};
+  if (doc.images && doc.images.length) {
+    const zip = await JSZip.loadAsync(epubBytes);
+    await Promise.all(doc.images.map(async path => {
+      const f = zip.file(path);
+      if (!f) return;
+      try {
+        const buf = await f.async('arraybuffer');
+        const ext = path.split('.').pop().toLowerCase();
+        const type = {jpg: 'image/jpeg', jpeg: 'image/jpeg', png: 'image/png', gif: 'image/gif',
+                      svg: 'image/svg+xml', webp: 'image/webp'}[ext] || '';
+        imageMap[path] = URL.createObjectURL(new Blob([buf], type ? {type} : {}));
+      } catch (e) {}
+    }));
+  }
+  const placeholder = /diora-img:([^"'\s)]+)/g;
+  const chapters = doc.chapters.map(c => c.replace(placeholder, (m, path) => imageMap[path] || ''));
+  return {title: doc.title, author: doc.author, toc: doc.toc, chapters, imageMap};
+}
+
 async function _getCachedBook(bookId) {
   try {
     const db = await _openBookCacheDb();
@@ -3754,22 +3855,29 @@ async function _openBookOnce(bookId) {
       currentPdfBuffer = plain;
     } else {
       currentPdfBuffer = null;
-      const result = await parseEpub(plain);
+      // Prepared before (same book file, same preparation code): only the images to unpack.
+      const prepared = await _getParsedBook(bookId, data_iv, key);
+      const result = prepared ? await _restoreParsedBook(prepared, plain) : await parseEpub(plain);
+      if (!prepared) {
+        // Kept for the next open once the book is on screen (encrypting takes a moment).
+        setTimeout(() => _setParsedBook(bookId, data_iv, key, result), 3000);
+      }
       title = result.title || title;
       author = result.author || author;
       toc = result.toc;
       currentImageMap = result.imageMap;
-      // Inject chapters progressively so the browser stays responsive on large books
-      const BATCH = 8;
-      for (let i = 0; i < result.chapters.length; i += BATCH) {
+      // Inject chapters progressively so the browser stays responsive on large books:
+      // a batch per ~50 ms of work (a fixed count waited a frame every 8 chapters).
+      const tmp = document.createElement('div');
+      for (let i = 0; i < result.chapters.length;) {
         const frag = document.createDocumentFragment();
-        const tmp = document.createElement('div');
-        for (let j = i; j < Math.min(i + BATCH, result.chapters.length); j++) {
-          tmp.innerHTML = result.chapters[j];
+        const start = performance.now();
+        do {
+          tmp.innerHTML = result.chapters[i++];
           while (tmp.firstChild) frag.appendChild(tmp.firstChild);
-        }
+        } while (i < result.chapters.length && performance.now() - start < 50);
         contentEl.appendChild(frag);
-        await new Promise(r => requestAnimationFrame(r));
+        if (i < result.chapters.length) await new Promise(r => setTimeout(r, 0));
       }
       // Used to decide whole-percent vs one-decimal display further down —
       // see _formatBookPercent().