perf(chart): precompute session usage timestamps before sorting

.jules/bolt.md

@@ -3,3 +3,9 @@
 **Learning:** `Date.parse(value)` returns the timestamp primitive directly, while `new Date(value).getTime()` also constructs a `Date` object. Both use the same ECMAScript string-parsing semantics for these call sites.
 
 **Action:** In frequently executed paths that only need a timestamp primitive, prefer `Date.parse(value)`. Treat the allocation reduction as a bounded micro-optimization unless a committed benchmark establishes a larger runtime effect.
+
+## 2026-09-09 - Avoid O(N log N) `Date.parse` in arrays.sort
+
+**Learning:** Array sorting with complex transformations in the comparison function (like `Date.parse`) calls the transformation function O(N log N) times. Pre-computing the value transforms the work to O(N) mapping + O(N log N) sorting.
+
+**Action:** Use a Schwartzian transform (map-sort-map) for expensive sort keys. Do not spread (`...item`) original objects in the wrapper, which causes O(N) shallow copies; wrap them instead (`{ original: item, parsedValue }`).

packages/web/src/components/dashboard/session-timeline-chart.tsx

@@ -67,18 +67,19 @@ function buildChartData(
   toolCalls: ToolCallPoint[],
   sessionStartedAt: string
 ): ChartDataItem[] {
-  const sortedUsage = [...usageTimeline].sort(
-    (a, b) => Date.parse(a.timestamp) - Date.parse(b.timestamp)
-  )
+  // Schwartzian transform: pre-compute parsed timestamps to avoid O(N log N) Date.parse calls during sort
+  const sortedUsage = usageTimeline
+    .map((usage) => ({ original: usage, parsedTimestamp: Date.parse(usage.timestamp) }))
+    .sort((a, b) => a.parsedTimestamp - b.parsedTimestamp)
+
   const sortedTools = [...toolCalls].sort(
     (a, b) => a.parsedTimestamp - b.parsedTimestamp
   )
 
   let toolIndex = 0
   const cumulativeToolCounts = new Map<string, number>()
 
-  return sortedUsage.map((usage) => {
-    const currentTimestamp = Date.parse(usage.timestamp)
+  return sortedUsage.map(({ original: usage, parsedTimestamp: currentTimestamp }) => {
 
     while (
       toolIndex < sortedTools.length &&