simplify-trace

name	simplify-trace
description	Summarize a large Chrome DevTools performance trace into a compact markdown report so it can be reasoned about without loading the whole file. Use when given a Chrome/DevTools/Performance-panel trace (a multi-MB `Trace-*.json` or `*.json` with `traceEvents`) and asked to find what is slow, what runs too often, long tasks, jank, or hot JS functions.

Simplify trace

Chrome DevTools performance traces are tens to hundreds of MB of JSON — far too large to read directly. This skill turns one into a few-KB markdown report that surfaces the work taking too long or happening too often.

Usage

Run the script on the trace file. It prints markdown to stdout (or --out FILE):

node skills/simplify-trace/scripts/simplify-trace.mjs <trace.json> [--top N] [--long-task-ms MS] [--window START-END] [--out report.md]

• --top N — rows per table (default 25).
• --long-task-ms MS — long-task threshold (default 50).
• --window START-END — scope the whole report to a time slice (offsets in ms from trace start).
• --only a,b / --except a,b / --all — pick which sections to emit.
• --match TEXT — case-insensitive filter for rows (event names, function names, URLs).
• --list — print the available section keys and exit.
• --out FILE — write to a file instead of stdout.
• Traces over ~500 MB: prefix node --max-old-space-size=8192.

Default to running with --out to a temp file for big traces, then read the report. Don't read the raw trace.

Sections

Default sections: summary, longtasks, events, frequent, functions, categories. Opt-in: timeline (per-second main-thread busy time — use to locate activity), network (resource/fetch waterfall with TTFB/duration/size), websocket (WebSocket lifecycle — /app/file doc sync). Run --list for the full set. Interrogate narrowly, e.g.:

# where is the activity? then window to it
node …/simplify-trace.mjs trace.json --only timeline
# what was the network/socket doing during the action?
node …/simplify-trace.mjs trace.json --only network,websocket --window 62000-69000 --match tldraw

The trace's metadata.startTime (ISO/UTC) anchors offset 0 to wall-clock, so trace offsets can be lined up against server logs (zero-cache, sync-worker) by timestamp. For an idle, network-quiet gap, the trace shows when but not why — add performance.mark()/console.timeStamp() in the client path and they appear on the trace timeline.

"What happens when I do X" traces

A recording of a single action (switch file, open menu) is mostly idle setup time, which dilutes the action across the whole trace. Window to the action instead:

1. Run once with no window. Note where activity is — the long tasks' offsets, or bucket main-thread busy-time per second with a quick inline script to find the active span.
2. Re-run with --window START-END around that span. All tables then describe only the action.

The recording artifact CpuProfiler::StartProfiling (the profiler turning on, ~50–60ms) is excluded from the long-task table, including when it's nested inside a RunTask. If long tasks shows "None", the action genuinely has no single blocking task — look at aggregate self time, GC, and animation-loop events instead.

What the report contains

• Header — event count, wall-clock span, sampled JS CPU time, and idle %.
• Long tasks — top-level tasks over the threshold (main-thread jank), with the time offset where each occurred.
• Hottest event types (self time) — where engine/browser time actually goes (layout, GC, paint, function calls), excluding time spent in nested children.
• Most frequent event types (count) — work happening too often.
• Hottest JS functions — bottom-up self time from the embedded V8 CPU profile, with file:line. Synthetic (idle)/(program) frames are excluded here (idle is in the header).
• Self time by category — high-level breakdown across trace categories.

How to read it

• A function high in self time is the actual cost; high total but low self means the cost is in its callees — follow the call tree.
• High count with low avg = death by a thousand cuts (often a reactive/render loop firing too often); investigate why it fires, not its per-call cost.
• Long tasks point at when jank happened; cross-reference the offset against what the user was doing.
• Minified names (r, Tg) come with a file:line — use it to locate the source.

The script only summarizes; it does not modify the trace.