diff --git a/lib/KOReaderSync/KOReaderDocumentId.cpp b/lib/KOReaderSync/KOReaderDocumentId.cpp
index efb18d1b..0d5ea9b3 100644
--- a/lib/KOReaderSync/KOReaderDocumentId.cpp
+++ b/lib/KOReaderSync/KOReaderDocumentId.cpp
@@ -4,6 +4,8 @@
 #include <Logging.h>
 #include <MD5Builder.h>
 
+#include <functional>
+
 namespace {
 // Extract filename from path (everything after last '/')
 std::string getFilename(const std::string& path) {
@@ -15,6 +17,130 @@ std::string getFilename(const std::string& path) {
 }
 }  // namespace
 
+std::string KOReaderDocumentId::getCacheFilePath(const std::string& filePath) {
+  // Mirror the Epub cache directory convention so the hash file shares the
+  // same per-book folder as other cached data.
+  return std::string("/.crosspoint/epub_") + std::to_string(std::hash<std::string>{}(filePath)) + "/koreader_docid.txt";
+}
+
+std::string KOReaderDocumentId::loadCachedHash(const std::string& cacheFilePath, const size_t fileSize,
+                                               const std::string& currentFingerprint) {
+  if (!Storage.exists(cacheFilePath.c_str())) {
+    return "";
+  }
+
+  const String content = Storage.readFile(cacheFilePath.c_str());
+  if (content.isEmpty()) {
+    return "";
+  }
+
+  // Format: "<filesize>:<fingerprint>\n<32-char-hex-hash>"
+  const int newlinePos = content.indexOf('\n');
+  if (newlinePos < 0) {
+    return "";
+  }
+
+  const String header = content.substring(0, newlinePos);
+  const int colonPos = header.indexOf(':');
+  if (colonPos < 0) {
+    LOG_DBG("KODoc", "Hash cache invalidated: header missing fingerprint");
+    return "";
+  }
+
+  const String sizeTok = header.substring(0, colonPos);
+  const String fpTok = header.substring(colonPos + 1);
+
+  // Validate the filesize token – it must consist of ASCII digits and parse
+  // correctly to the expected size.
+  bool digitsOnly = true;
+  for (size_t i = 0; i < sizeTok.length(); ++i) {
+    const char ch = sizeTok[i];
+    if (ch < '0' || ch > '9') {
+      digitsOnly = false;
+      break;
+    }
+  }
+  if (!digitsOnly) {
+    LOG_DBG("KODoc", "Hash cache invalidated: size token not numeric ('%s')", sizeTok.c_str());
+    return "";
+  }
+
+  const long parsed = sizeTok.toInt();
+  if (parsed < 0) {
+    LOG_DBG("KODoc", "Hash cache invalidated: size token parse error ('%s')", sizeTok.c_str());
+    return "";
+  }
+  const size_t cachedSize = static_cast<size_t>(parsed);
+  if (cachedSize != fileSize) {
+    LOG_DBG("KODoc", "Hash cache invalidated: file size or fingerprint changed (%zu -> %zu)", cachedSize, fileSize);
+    return "";
+  }
+
+  // Validate stored fingerprint format (8 hex characters)
+  if (fpTok.length() != 8) {
+    LOG_DBG("KODoc", "Hash cache invalidated: bad fingerprint length (%zu)", fpTok.length());
+    return "";
+  }
+  for (size_t i = 0; i < fpTok.length(); ++i) {
+    char c = fpTok[i];
+    bool hex = (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F');
+    if (!hex) {
+      LOG_DBG("KODoc", "Hash cache invalidated: non-hex character '%c' in fingerprint", c);
+      return "";
+    }
+  }
+
+  {
+    String currentFpStr(currentFingerprint.c_str());
+    if (fpTok != currentFpStr) {
+      LOG_DBG("KODoc", "Hash cache invalidated: fingerprint changed (%s != %s)", fpTok.c_str(),
+              currentFingerprint.c_str());
+      return "";
+    }
+  }
+
+  std::string hash = content.substring(newlinePos + 1).c_str();
+  // Trim any trailing whitespace / line endings
+  while (!hash.empty() && (hash.back() == '\n' || hash.back() == '\r' || hash.back() == ' ')) {
+    hash.pop_back();
+  }
+
+  // Hash must be exactly 32 hex characters.
+  if (hash.size() != 32) {
+    LOG_DBG("KODoc", "Hash cache invalidated: wrong hash length (%zu)", hash.size());
+    return "";
+  }
+  for (char c : hash) {
+    if (!((c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F'))) {
+      LOG_DBG("KODoc", "Hash cache invalidated: non-hex character '%c' in hash", c);
+      return "";
+    }
+  }
+
+  LOG_DBG("KODoc", "Hash cache hit: %s", hash.c_str());
+  return hash;
+}
+
+void KOReaderDocumentId::saveCachedHash(const std::string& cacheFilePath, const size_t fileSize,
+                                        const std::string& fingerprint, const std::string& hash) {
+  // Ensure the book's cache directory exists before writing
+  const size_t lastSlash = cacheFilePath.rfind('/');
+  if (lastSlash != std::string::npos) {
+    Storage.ensureDirectoryExists(cacheFilePath.substr(0, lastSlash).c_str());
+  }
+
+  // Format: "<filesize>:<fingerprint>\n<hash>"
+  String content(std::to_string(fileSize).c_str());
+  content += ':';
+  content += fingerprint.c_str();
+  content += '\n';
+  content += hash.c_str();
+
+  if (!Storage.writeFile(cacheFilePath.c_str(), content)) {
+    LOG_DBG("KODoc", "Failed to write hash cache to %s", cacheFilePath.c_str());
+  }
+}
+
 std::string KOReaderDocumentId::calculateFromFilename(const std::string& filePath) {
   const std::string filename = getFilename(filePath);
   if (filename.empty()) {
@@ -49,6 +175,30 @@ std::string KOReaderDocumentId::calculate(const std::string& filePath) {
   }
 
   const size_t fileSize = file.fileSize();
+
+  // Compute a lightweight fingerprint from the file's modification time.
+  // The underlying FsFile API provides getModifyDateTime which returns two
+  // packed 16-bit values (date and time).  Concatenate these as eight hex
+  // digits to produce the token stored in the cache header.
+  uint16_t date = 0, time = 0;
+  if (!file.getModifyDateTime(&date, &time)) {
+    // If timestamp isn't available for some reason, fall back to a sentinel.
+    date = 0;
+    time = 0;
+  }
+  char fpBuf[9];
+  // two 16-bit numbers => 4 hex digits each
+  sprintf(fpBuf, "%04x%04x", date, time);
+  const std::string fingerprintTok(fpBuf);
+
+  // Return persisted hash if the file size and fingerprint haven't changed.
+  const std::string cacheFilePath = getCacheFilePath(filePath);
+  const std::string cached = loadCachedHash(cacheFilePath, fileSize, fingerprintTok);
+  if (!cached.empty()) {
+    file.close();
+    return cached;
+  }
+
   LOG_DBG("KODoc", "Calculating hash for file: %s (size: %zu)", filePath.c_str(), fileSize);
 
   // Initialize MD5 builder
@@ -92,5 +242,7 @@ std::string KOReaderDocumentId::calculate(const std::string& filePath) {
 
   LOG_DBG("KODoc", "Hash calculated: %s (from %zu bytes)", result.c_str(), totalBytesRead);
 
+  saveCachedHash(cacheFilePath, fileSize, fingerprintTok, result);
+
   return result;
 }
diff --git a/lib/KOReaderSync/KOReaderDocumentId.h b/lib/KOReaderSync/KOReaderDocumentId.h
index 2b6189e2..5f226eb5 100644
--- a/lib/KOReaderSync/KOReaderDocumentId.h
+++ b/lib/KOReaderSync/KOReaderDocumentId.h
@@ -42,4 +42,31 @@ class KOReaderDocumentId {
 
   // Calculate offset for index i: 1024 << (2*i)
   static size_t getOffset(int i);
+
+  // Hash cache helpers
+  // Returns the path to the per-book cache file that stores the precomputed hash.
+  // Uses the same directory convention as the Epub cache (/.crosspoint/epub_<hash>/).
+  static std::string getCacheFilePath(const std::string& filePath);
+
+  // Returns the cached hash if the file size and fingerprint match, or empty
+  // string on miss/invalidation.
+  //
+  // The fingerprint is derived from the file's modification timestamp.  We
+  // call `FsFile::getModifyDateTime` to retrieve two 16‑bit packed values
+  // supplied by the filesystem: one for the date and one for the time.  These
+  // are concatenated and represented as eight hexadecimal digits in the form
+  // <date><time> (high 16 bits = packed date, low 16 bits = packed time).
+  //
+  // The resulting string serves as a lightweight change signal; any modification
+  // to the file's mtime will alter the packed date/time combo and invalidate
+  // the cache entry.  Since the full document hash is expensive to compute,
+  // using the packed timestamp gives us a quick way to detect modifications
+  // without reading file contents.
+  static std::string loadCachedHash(const std::string& cacheFilePath, size_t fileSize,
+                                    const std::string& currentFingerprint);
+
+  // Persists the computed hash alongside the file size and fingerprint (the
+  // modification-timestamp token) used to generate it.
+  static void saveCachedHash(const std::string& cacheFilePath, size_t fileSize, const std::string& fingerprint,
+                             const std::string& hash);
 };