perf(ui): calm large-queue live updates

Throttle visible download snapshots at 500 ms from 250 queue items, align speed visualizations to the same cadence, preserve small-queue responsiveness, and retain the stronger protection for very large queues.

src/main/download-manager.ts

@@ -6547,15 +6547,13 @@ export class DownloadManager extends EventEmitter {
     if (this.stateEmitTimer) {
       return;
     }
-    const itemCount = this.itemCount;
-    const emitDelay = this.session.running
-      ? itemCount >= 1500
-        ? 700
-        : itemCount >= 700
-          ? 500
-          : itemCount >= 250
-            ? 300
-            : 150
+    const itemCount = this.itemCount;
+    const emitDelay = this.session.running
+      ? itemCount >= 1500
+        ? 700
+        : itemCount >= 250
+          ? 500
+          : 150
       : 200;
     this.stateEmitTimer = setTimeout(() => {
       this.stateEmitTimer = null;
@@ -10566,14 +10564,12 @@ export class DownloadManager extends EventEmitter {
         let windowStarted = nowMs();
         let lastPackageLogAt = 0;
         let lastLoggedPercent = -1;
-        const itemCount = this.itemCount;
-        const uiUpdateIntervalMs = itemCount >= 1500
-          ? 500
-          : itemCount >= 700
-            ? 350
-            : itemCount >= 250
-              ? 220
-              : 120;
+        const itemCount = this.itemCount;
+        const uiUpdateIntervalMs = itemCount >= 1500
+          ? 700
+          : itemCount >= 250
+            ? 500
+            : 120;
         let lastUiEmitAt = 0;
         const stallTimeoutMs = getDownloadStallTimeoutMs();
         const drainTimeoutMs = Math.max(30000, Math.min(300000, stallTimeoutMs > 0 ? stallTimeoutMs * 12 : 120000));

src/renderer/App.tsx

@@ -880,7 +880,7 @@ const historyRetentionLabels: Record<RendererSettings["historyRetentionMode"], s
 const AUTO_RENDER_PACKAGE_LIMIT = 260;
 
 export function getSnapshotRenderDelay(itemCount: number, running: boolean, activeTab: MainView): number {
-  let delay = itemCount >= 700 ? 0 : itemCount >= 250 ? 50 : 100;
+  let delay = itemCount >= 250 ? 0 : 100;
   if (!running) delay = Math.min(delay, 200);
   if (activeTab !== "downloads") delay = Math.max(delay, 800);
   return delay;
@@ -1272,7 +1272,7 @@ const BandwidthChart = memo(function BandwidthChart({ running, paused, speedHist
     const reducedMotion = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
     const interval = setInterval(() => {
       drawChart();
-    }, reducedMotion ? 1000 : 250);
+    }, reducedMotion ? 1000 : 500);
     return () => clearInterval(interval);
   }, [drawChart, running, paused]);
 
@@ -1374,7 +1374,7 @@ const DownloadSpeedSparkline = memo(function DownloadSpeedSparkline({ speedBps,
       draw();
     };
 
-    const id = window.setInterval(tick, 250);
+    const id = window.setInterval(tick, 500);
     return () => window.clearInterval(id);
   }, []);