simplify-trace

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.