diff --git a/main.js b/main.js
index c693662..c25baf1 100644
--- a/main.js
+++ b/main.js
@@ -4,6 +4,7 @@ const fs = require('fs-extra');
 
 const { processFolder } = require('./services/folderProcessor');
 const { processZip } = require('./services/zipProcessor');
+const { setCacheDir, clearCache } = require('./services/destinationScanner');
 const {
   resolveUnroutedDir,
   listUnroutedFiles,
@@ -127,6 +128,7 @@ Menu.setApplicationMenu(Menu.buildFromTemplate([
 
 app.whenReady().then(async () => {
   await ensureRuntimeDirectory();
+  setCacheDir(getRuntimeDirectory());
   const persistedDestination = await loadPersistedDestination();
   config = buildConfig(persistedDestination);
   createWindow();
@@ -224,9 +226,11 @@ ipcMain.handle('update-destination', async (_event, payload) => {
 
   config = buildConfig(destination);
   await persistDestination(config.destination);
+  await clearCache();
   return { destination: config.destination };
 });
 
+
 ipcMain.handle('open-unrouted-folder', async () => {
   const unroutedDir = await resolveUnroutedDir();
   const openResult = await shell.openPath(unroutedDir);
diff --git a/renderer/renderer.js b/renderer/renderer.js
index 6041106..6b639d5 100644
--- a/renderer/renderer.js
+++ b/renderer/renderer.js
@@ -35,19 +35,34 @@ function hideProgress() {
 let _indexFilesFound = 0;
 
 function updateProgress({ phase, current, total, file, scanned, folder }) {
-  if (phase === 'scan') {
+  if (phase === 'index-cache') {
+    // folderIndex dalla cache — avvia scansione veloce dei file
+    progressBar.max = total || 1;
+    progressBar.value = 0;
+    progressLabel.textContent = 'Struttura dalla cache — ricerca file CAD in corso...';
+    return;
+  } else if (phase === 'scan') {
     _indexFilesFound = 0;
     progressLabel.textContent = 'Scansione file sorgente...';
     progressBar.removeAttribute('value');
   } else if (phase === 'index-dest') {
+    // scansione completa (nessuna cache)
     progressBar.max = total;
     progressBar.value = current;
     const filesInfo = _indexFilesFound > 0 ? `  —  ${_indexFilesFound} file CAD trovati` : '';
     progressLabel.textContent = `Analisi destinazione: ${current} / ${total}${folder ? `  —  ${folder}` : ''}${filesInfo}`;
   } else if (phase === 'index-dup') {
-    _indexFilesFound = scanned;
-    const dirInfo = progressBar.max > 0 ? `${progressBar.value} / ${progressBar.max}  —  ` : '';
-    progressLabel.textContent = `Analisi destinazione: ${dirInfo}${scanned} file CAD trovati`;
+    if (current !== undefined && total !== undefined) {
+      // scansione veloce (cache attiva): current/total = cartelle processate
+      progressBar.max = total;
+      progressBar.value = current;
+      progressLabel.textContent = `Ricerca file CAD: ${current} / ${total} cartelle`;
+    } else {
+      // scansione completa: scanned = contatore file trovati
+      _indexFilesFound = scanned;
+      const dirInfo = progressBar.max > 0 ? `${progressBar.value} / ${progressBar.max}  —  ` : '';
+      progressLabel.textContent = `Analisi destinazione: ${dirInfo}${scanned} file CAD trovati`;
+    }
   } else if (phase === 'copy') {
     progressBar.max = total;
     progressBar.value = current;
@@ -349,6 +364,7 @@ clearPreviewBtn.addEventListener('click', async () => {
   }
 });
 
+
 closePreviewBtn.addEventListener('click', hidePreview);
 previewOverlay.addEventListener('click', (event) => {
   if (event.target === previewOverlay) {
diff --git a/services/destinationScanner.js b/services/destinationScanner.js
index 0cea3de..a31ef51 100644
--- a/services/destinationScanner.js
+++ b/services/destinationScanner.js
@@ -4,21 +4,64 @@ const { getCadInfo } = require('./router');
 
 const CODE_FOLDER_REGEX = /^\d{3}$/;
 const MAX_SCAN_DEPTH = 6;
+const CACHE_FILENAME = 'dest-index.cache.json';
+
+let _cacheDir = null;
+
+function setCacheDir(dir) {
+  _cacheDir = dir;
+}
 
 function toCadKey(cadInfo) {
   return String(cadInfo?.key || '').toLowerCase();
 }
 
-/**
- * Scansiona la cartella di destinazione in un unico passaggio parallelo e
- * costruisce contemporaneamente:
- *   - folderIndex: Map<group3digits, fullPath[]>  (per il routing)
- *   - cadKeyIndex: Set<cadKey>                    (per il rilevamento duplicati)
- *
- * Legge prima le sottocartelle di primo livello (1 sola readdir) per ottenere
- * il totale e mostrare una barra determinata durante la scansione.
- */
-async function buildDestinationIndexes(destinationRoot, onProgress) {
+function getCacheFilePath() {
+  return _cacheDir ? path.join(_cacheDir, CACHE_FILENAME) : null;
+}
+
+// ── Cache su disco (solo folderIndex — struttura cartelle) ─────────────────
+
+async function tryLoadFolderIndexCache(destinationRoot) {
+  const cacheFile = getCacheFilePath();
+  if (!cacheFile) return null;
+
+  try {
+    const raw = await fs.readJson(cacheFile);
+    if (raw?.destination !== destinationRoot) return null;
+    return { folderIndex: new Map(raw.folderIndex || []), savedAt: raw.savedAt };
+  } catch {
+    return null;
+  }
+}
+
+async function saveFolderIndexCache(destinationRoot, folderIndex) {
+  const cacheFile = getCacheFilePath();
+  if (!cacheFile) return;
+  try {
+    await fs.writeJson(cacheFile, {
+      destination: destinationRoot,
+      savedAt: new Date().toISOString(),
+      folderIndex: [...folderIndex.entries()],
+    });
+  } catch {
+    // cache non critica
+  }
+}
+
+async function clearCache() {
+  const cacheFile = getCacheFilePath();
+  if (!cacheFile) return;
+  try {
+    await fs.remove(cacheFile);
+  } catch {
+    // ignora
+  }
+}
+
+// ── Scansione completa (folderIndex + cadKeyIndex in un unico passaggio) ───
+
+async function fullScan(destinationRoot, onProgress) {
   const folderIndex = new Map();
   const cadKeyIndex = new Set();
 
@@ -26,7 +69,6 @@ async function buildDestinationIndexes(destinationRoot, onProgress) {
     return { folderIndex, cadKeyIndex };
   }
 
-  // Lettura di primo livello: 1 sola chiamata per avere il totale
   let topEntries;
   try {
     topEntries = await fs.readdir(destinationRoot, { withFileTypes: true });
@@ -42,7 +84,6 @@ async function buildDestinationIndexes(destinationRoot, onProgress) {
   let completed = 0;
   let scannedFiles = 0;
 
-  // Registra i file CAD al primo livello (non cartelle)
   for (const entry of topEntries) {
     if (!entry.isFile()) continue;
     const cadInfo = getCadInfo(entry.name);
@@ -52,11 +93,8 @@ async function buildDestinationIndexes(destinationRoot, onProgress) {
     }
   }
 
-  // Walk ricorsivo del sottoalbero di una cartella di primo livello
   async function walkSubtree(dirPath, depth) {
-    if (depth > MAX_SCAN_DEPTH) {
-      return;
-    }
+    if (depth > MAX_SCAN_DEPTH) return;
 
     let entries;
     try {
@@ -90,7 +128,6 @@ async function buildDestinationIndexes(destinationRoot, onProgress) {
     await Promise.all(subdirPromises);
   }
 
-  // Processa tutte le cartelle di primo livello in parallelo
   await Promise.all(
     topDirs.map(async ({ name, fullPath }) => {
       onProgress?.({ phase: 'index-dest', current: completed, total, folder: name });
@@ -111,4 +148,117 @@ async function buildDestinationIndexes(destinationRoot, onProgress) {
   return { folderIndex, cadKeyIndex };
 }
 
-module.exports = { buildDestinationIndexes, toCadKey };
+// ── Scansione veloce dei file (usa il folderIndex già noto dalla cache) ────
+//
+// Invece di traversare l'intero albero, legge direttamente SOLO le cartelle
+// a 3 cifre già note. Questo riduce drasticamente le chiamate readdir su SMB.
+
+async function scanCadKeysFromFolderIndex(folderIndex, onProgress) {
+  const cadKeyIndex = new Set();
+  const allFolderPaths = [...folderIndex.values()].flat();
+  const total = allFolderPaths.length;
+  let completed = 0;
+
+  await Promise.all(
+    allFolderPaths.map(async (folderPath) => {
+      for (const sub of ['A', 'M_B']) {
+        const subPath = path.join(folderPath, sub);
+        let entries;
+        try {
+          entries = await fs.readdir(subPath, { withFileTypes: true });
+        } catch {
+          continue;
+        }
+        for (const entry of entries) {
+          if (!entry.isFile()) continue;
+          const cadInfo = getCadInfo(entry.name);
+          if (cadInfo) {
+            cadKeyIndex.add(toCadKey(cadInfo));
+          }
+        }
+      }
+
+      completed += 1;
+      onProgress?.({ phase: 'index-dup', current: completed, total });
+    })
+  );
+
+  return cadKeyIndex;
+}
+
+// ── Entry point ────────────────────────────────────────────────────────────
+
+async function isCacheStillValid(destinationRoot, cached) {
+  // Controlla le cartelle top-level E le cartelle a 3 cifre al loro interno.
+  // Costa 1 + N_top_level readdir — rileva nuove cartelle aggiunte da colleghi.
+  let currentEntries;
+  try {
+    currentEntries = await fs.readdir(destinationRoot, { withFileTypes: true });
+  } catch {
+    return false;
+  }
+
+  const currentDirNames = new Set(
+    currentEntries.filter((e) => e.isDirectory()).map((e) => e.name)
+  );
+
+  // Ricostruisce: topLevelDir → Set<group> dalla cache
+  const cachedGroupsByTopDir = new Map();
+  for (const [group, paths] of cached.folderIndex.entries()) {
+    for (const p of paths) {
+      const topDir = path.relative(destinationRoot, p).split(path.sep)[0];
+      if (!cachedGroupsByTopDir.has(topDir)) cachedGroupsByTopDir.set(topDir, new Set());
+      cachedGroupsByTopDir.get(topDir).add(group);
+    }
+  }
+
+  const cachedDirNames = new Set(cachedGroupsByTopDir.keys());
+
+  // Controlla top-level
+  for (const name of currentDirNames) {
+    if (!cachedDirNames.has(name)) return false;
+  }
+  for (const name of cachedDirNames) {
+    if (!currentDirNames.has(name)) return false;
+  }
+
+  // Controlla cartelle a 3 cifre dentro ogni top-level (solo nuove aggiunte)
+  for (const topDir of currentDirNames) {
+    let subEntries;
+    try {
+      subEntries = await fs.readdir(path.join(destinationRoot, topDir), { withFileTypes: true });
+    } catch {
+      return false;
+    }
+    const cachedGroups = cachedGroupsByTopDir.get(topDir) || new Set();
+    for (const e of subEntries) {
+      if (e.isDirectory() && CODE_FOLDER_REGEX.test(e.name) && !cachedGroups.has(e.name)) {
+        return false; // nuova cartella a 3 cifre rilevata
+      }
+    }
+  }
+
+  return true;
+}
+
+async function buildDestinationIndexes(destinationRoot, onProgress) {
+  if (!destinationRoot) {
+    return { folderIndex: new Map(), cadKeyIndex: new Set() };
+  }
+
+  const cached = await tryLoadFolderIndexCache(destinationRoot);
+
+  if (cached && (await isCacheStillValid(destinationRoot, cached))) {
+    // folderIndex dalla cache (istantaneo), cadKeyIndex sempre fresco
+    onProgress?.({ phase: 'index-cache', savedAt: cached.savedAt, total: cached.folderIndex.size });
+    const cadKeyIndex = await scanCadKeysFromFolderIndex(cached.folderIndex, onProgress);
+    return { folderIndex: cached.folderIndex, cadKeyIndex };
+  }
+
+  // Prima volta, cache invalidata o struttura cambiata: scansione completa
+  const result = await fullScan(destinationRoot, onProgress);
+  await saveFolderIndexCache(destinationRoot, result.folderIndex);
+  return result;
+}
+
+module.exports = { buildDestinationIndexes, setCacheDir, clearCache, toCadKey };
diff --git a/services/router.js b/services/router.js
index 502ce21..8e9f013 100644
--- a/services/router.js
+++ b/services/router.js
@@ -16,6 +16,9 @@ function getCadInfo(filename) {
   const version = match[3] || '';
   const digits = rawCode.replace(/\D/g, '');
   const routingGroup = digits.length >= 5 ? digits.slice(2, 5) : null;
+  const subfolder = digits.length >= 2
+    ? (digits[1] === '1' ? 'A' : digits[1] === '2' ? 'M_B' : null)
+    : null;
 
   return {
     code: rawCode,
@@ -24,6 +27,7 @@ function getCadInfo(filename) {
     version,
     key: `${rawCode}.${type}`,
     routingGroup,
+    subfolder,
   };
 }
 
@@ -57,7 +61,14 @@ function getDestinationDecision(filename, config, destinationIndex) {
     };
   }
 
-  return { destination: candidates[0], reason: null };
+  if (!cadInfo.subfolder) {
+    return {
+      destination: null,
+      reason: `Prefisso non riconosciuto (${cadInfo.digits[1]}): atteso 91 (→ A) o 92 (→ M_B)`,
+    };
+  }
+
+  return { destination: path.join(candidates[0], cadInfo.subfolder), reason: null };
 }
 
 function findDestination(filename, config, destinationIndex) {