diff --git a/eslint.config.mjs b/eslint.config.mjs
index 6759a1c..4a53a64 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -20,6 +20,7 @@ export default [
         clearTimeout: 'readonly',
         setInterval: 'readonly',
         clearInterval: 'readonly',
+        setImmediate: 'readonly',
         Buffer: 'readonly',
         URL: 'readonly',
         fetch: 'readonly',
diff --git a/main.js b/main.js
index 6c7f054..4a4feaf 100644
--- a/main.js
+++ b/main.js
@@ -35,10 +35,36 @@ function getDebugLogPath() {
   return path.join(baseDir, 'upload-debug.log');
 }
 
+// Buffered async writer: debugLog is called hundreds of times per second during
+// busy uploads (unhandledRejection traces, progress transitions, folder-monitor
+// events). Sync appendFileSync per call blocked the main event loop. We now
+// queue lines in memory and flush on a short interval / on process exit.
+const _debugLogBuffer = [];
+let _debugLogFlushTimer = null;
+let _debugLogWriting = false;
+
+function _flushDebugLog() {
+  if (_debugLogWriting || _debugLogBuffer.length === 0) return;
+  const chunk = _debugLogBuffer.join('');
+  _debugLogBuffer.length = 0;
+  _debugLogWriting = true;
+  fs.appendFile(getDebugLogPath(), chunk, 'utf-8', () => {
+    _debugLogWriting = false;
+    // If more lines arrived during the write, flush them next tick.
+    if (_debugLogBuffer.length) setImmediate(_flushDebugLog);
+  });
+}
+
 function debugLog(msg) {
   try {
     const ts = new Date().toISOString();
-    fs.appendFileSync(getDebugLogPath(), `[${ts}] ${msg}\n`, 'utf-8');
+    _debugLogBuffer.push(`[${ts}] ${msg}\n`);
+    if (!_debugLogFlushTimer) {
+      _debugLogFlushTimer = setTimeout(() => {
+        _debugLogFlushTimer = null;
+        _flushDebugLog();
+      }, 500);
+    }
   } catch {}
 }
 
@@ -725,6 +751,13 @@ app.on('before-quit', () => {
     destroyCaptureWindow();
   } catch {}
   destroyDropTargetWindow();
+  // Flush pending debug-log buffer synchronously so no lines are lost.
+  try {
+    if (_debugLogBuffer.length) {
+      fs.appendFileSync(getDebugLogPath(), _debugLogBuffer.join(''), 'utf-8');
+      _debugLogBuffer.length = 0;
+    }
+  } catch {}
 });
 
 // --- IPC Handlers ---
diff --git a/renderer/app.js b/renderer/app.js
index ce77638..6c7dfaf 100644
--- a/renderer/app.js
+++ b/renderer/app.js
@@ -60,6 +60,9 @@ const selectedRecentIds = new Set();
 // Maintained incrementally — avoids O(n) filter() scans every 250ms in the status bar.
 let _sessionDoneCount = 0;
 let _sessionErrorCount = 0;
+// O(1) dedup for maybeAddSessionFile (was O(n) sessionFilesData.some).
+// Huge with thousands of rows × thousands of incoming results.
+const _sessionFileKeys = new Set();
 
 // --- Init ---
 async function init() {
@@ -93,9 +96,13 @@ async function init() {
   window.api.onUpdateAvailable(showUpdateBanner);
   window.api.onUpdateProgress(handleUpdateProgress);
 
-  // Upload event listeners — with debug logging to file
+  // Upload event listeners — debug log only on state transitions; the 'uploading'
+  // tick fires 4×/sec per active job and an IPC roundtrip per event would
+  // backlog the renderer↔main channel with hundreds of messages/sec.
   window.api.onUploadProgress((data) => {
-    window.api.debugLog('RX upload-progress: ' + data.status + ' ' + data.hoster + ' ' + (data.fileName || ''));
+    if (data.status !== 'uploading') {
+      window.api.debugLog('RX upload-progress: ' + data.status + ' ' + data.hoster + ' ' + (data.fileName || ''));
+    }
     handleProgress(data);
   });
   window.api.onUploadBatchDone((data) => {
@@ -103,7 +110,10 @@ async function init() {
     handleBatchDone(data);
   });
   window.api.onUploadStats((data) => {
-    window.api.debugLog('RX upload-stats: state=' + data.state + ' active=' + data.activeJobs);
+    // Stats fire every second per upload session — skip while uploading.
+    if (data.state !== 'uploading') {
+      window.api.debugLog('RX upload-stats: state=' + data.state + ' active=' + data.activeJobs);
+    }
     handleStats(data);
   });
   window.api.onShutdownCountdown(handleShutdownCountdown);
@@ -1005,11 +1015,18 @@ function _renderVirtualRows(tbody) {
   tbody.innerHTML = html;
 }
 
+// Coalesce rapid scroll events (a fast trackpad fling fires dozens) into one
+// render per frame. rAF keeps the scroll thread cheap.
+let _queueScrollQueued = false;
 function _onQueueScroll() {
-  if (_sortedJobsCache.length >= 200) {
+  if (_queueScrollQueued) return;
+  if (_sortedJobsCache.length < 200) return;
+  _queueScrollQueued = true;
+  requestAnimationFrame(() => {
+    _queueScrollQueued = false;
     const tbody = document.getElementById('queueBody');
     if (tbody) _renderVirtualRows(tbody);
-  }
+  });
 }
 
 const _collatorDE = new Intl.Collator('de', { sensitivity: 'base', numeric: true });
@@ -1179,6 +1196,7 @@ function deleteSelectedRecentFiles() {
   sessionFilesData = sessionFilesData.filter(r => {
     if (!selectedRecentIds.has(r.order)) return true;
     if (r.isError) removedErr++; else removedDone++;
+    _sessionFileKeys.delete(`${r.link}\u0001${r.filename}\u0001${r.host}`);
     return false;
   });
   _sessionDoneCount = Math.max(0, _sessionDoneCount - removedDone);
@@ -1191,6 +1209,7 @@ function clearAllRecentFiles() {
   if (sessionFilesData.length === 0) return;
   if (!confirm(`Wirklich alle ${sessionFilesData.length} Links aus diesem Panel entfernen?`)) return;
   sessionFilesData = [];
+  _sessionFileKeys.clear();
   _sessionDoneCount = 0;
   _sessionErrorCount = 0;
   selectedRecentIds.clear();
@@ -1949,7 +1968,9 @@ function maybeAddSessionFile(job) {
   if (job.status === 'done' && job.result) {
     const link = job.result.download_url || job.result.embed_url || '';
     if (!link) return;
-    if (!sessionFilesData.some((row) => row.link === link && row.filename === job.fileName && row.host === job.hoster)) {
+    const dedupKey = `${link}\u0001${job.fileName}\u0001${job.hoster}`;
+    if (!_sessionFileKeys.has(dedupKey)) {
+      _sessionFileKeys.add(dedupKey);
       sessionFilesData.push({
         date: dt.text,
         dateTs: dt.ts,