diff --git a/.jules/bolt.md b/.jules/bolt.md
index 7985023..549c99f 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -41,3 +41,6 @@
 ## 2024-05-15 - Fast cache keys using integer math
 **Learning:** Calling `new Date().toISOString().slice()` in hot paths (like `cached()` key generation during a backtest loop) adds up significantly (~1ms per thousand calls).
 **Action:** When grouping cache keys by day, use fast integer math `Math.floor(unixSecs / 86400)` instead of string-based date allocations to maintain identical semantics while avoiding object instantiation.
+## 2024-05-20 - Fast O(log n) lookups for day boundaries in time-series arrays
+**Learning:** `lastSessionOnly` in `web/server/index.ts` previously used a chained `bars.filter` with `new Date(b.time * 1000).toISOString().slice(0, 10)` to keep only the bars from the last calendar day. This resulted in O(N) array traversal with extremely heavy Date and String allocations for every bar, causing significant performance overhead on large historical ranges.
+**Action:** Replace `Date` and `toISOString()` string allocations with fast integer math (`Math.floor(time / 86400) * 86400`) to find the UTC day boundary in seconds. Since the time-series arrays are chronologically sorted, use O(log n) binary search (`findFirstBarIndex`) to locate the first matching bar and `.slice()` the rest, completely eliminating the allocation bottleneck.
diff --git a/web/server/index.ts b/web/server/index.ts
index 34c54ac..c6484bc 100644
--- a/web/server/index.ts
+++ b/web/server/index.ts
@@ -16,6 +16,7 @@ import { getDb } from "../../src/storage/db.js";
 import {
   getStockOhlcv,
   getIndexOhlcv,
+  findFirstBarIndex,
   type Bar,
   type Resolution,
 } from "../../src/data/sources/dnsePublic.js";
@@ -272,8 +273,11 @@ function rangeToPlan(range: RangeKey): { resolution: Resolution; fromSec: number
 /** For a 1D range, keep only bars from the latest calendar day present. */
 function lastSessionOnly(bars: Bar[]): Bar[] {
   if (!bars.length) return bars;
-  const lastDay = new Date(bars[bars.length - 1]!.time * 1000).toISOString().slice(0, 10);
-  return bars.filter((b) => new Date(b.time * 1000).toISOString().slice(0, 10) === lastDay);
+  // Use O(log n) binary search on chronologically sorted time-series arrays
+  // rather than an O(n) array filter with string date allocations.
+  const lastDayStartSec = Math.floor(bars[bars.length - 1]!.time / 86400) * 86400;
+  const idx = findFirstBarIndex(bars, lastDayStartSec);
+  return idx === -1 ? bars : bars.slice(idx);
 }
 
 // ---------------------------------------------------------------------------