profile-browsing

name

profile-browsing

description

Profile app performance while browsing, collecting Web Vitals and React rerender data via react-scan. Orchestrates parallel profiler subagents via playwright-cli to capture navigation timing, long tasks, layout shifts, LCP, React commit counts, render bursts, and per-component render data. Use when profiling browsing performance, finding bottlenecks, diagnosing excessive rerenders, or auditing page performance.

Profile Browsing Performance

Two-layer profiling: browser-level symptoms (Web Vitals, long tasks, scroll jank) and React-level diagnosis (commit counts, render bursts, per-component render data from react-scan). Each profiler subagent runs in its own browser session and context window.

Prerequisites

• Dev server running at https://bitsocial.localhost (yarn start via Portless)
• playwright-cli installed (npm install -g @playwright/cli@latest)

IMPORTANT: The orchestrator (you) is responsible for ensuring exactly one dev server is running. Profiler subagents must never start a dev server themselves.

react-scan and element-source (already configured)

The app exposes performance and source-inspection helpers in about/src/lib/react-scan.ts. In dev mode it:

• enables react-scan
• exposes window.__getReactScanReport() for programmatic render data
• exposes window.__ELEMENT_SOURCE__ for DOM-to-source resolution

The profiler's addInitScript sets window.__PROFILING__ = true before the app loads, which tells react-scan to disable its toolbar and sounds during automated runs.

Step 0: Ensure the Dev Server Is Running

Before spawning any profiler subagents, verify exactly one dev server is available:

curl -sf https://bitsocial.localhost -o /dev/null && echo "OK" || echo "NOT RUNNING"

• If OK: proceed to Step 1.
• If NOT RUNNING: start one instance with yarn start in the background, then poll until it responds. Do not start more than one.

Step 1: Define Route Batches

Split routes into batches of 2 to 4 for parallel profiling.

Default batches for this app:

Batch	Session	Routes	Focus
1	prof-1	/, /docs	Home page plus docs content
2	prof-2	/apps, /about	App directory plus about page
3	prof-3	/blog, /stats	Secondary content plus stats page

Adjust batches to match the routes relevant to the change under investigation.

Step 2: Spawn Profiler Subagents

Read the profiler subagent definition at .cursor/agents/profiler.md. Then spawn one profiler subagent per batch in parallel using Cursor's current delegation tool.

For each batch, include:

• agent: "profiler"
• session name: prof-N
• routes to profile
• any non-default app URL or extra profiling constraints

Spawn up to 4 subagents simultaneously. Each one opens its own browser session, navigates routes, scrolls, collects both Web Vitals and react-scan data per route, and returns a structured issues list.

Parallel is faster but can skew timing results under heavy machine load. For precise measurements, run the batches sequentially.

Step 3: Merge Results

Collect structured output from each subagent and merge:

1. Concatenate all Critical, Warning, React Rerenders, Scroll Jank, and Info items.
2. Combine per-view summary tables into one.
3. Merge react-scan component data across routes.
4. Deduplicate shared issues.
5. Sort by severity with Critical first.

Step 4: Final Report

## Performance Profile Results

### Critical
- [metric]: [value] at [route] — [what likely needs fixing]

### Warning
- [metric]: [value] at [route] — [what likely needs fixing]

### React Rerenders
- [route]: [N] commits during load, [M] during scroll — [likely cause]
- Render bursts detected at [routes] — suggests cascading state updates
- Top rerendering components (react-scan):
  - [ComponentName]: [total count] renders across [routes], [time]ms total
  - [ComponentName]: [total count] renders across [routes], [time]ms total

### Scroll Jank
- [route]: [N] long tasks during scroll (max [X]ms), [M] React commits — [likely cause]

### Info
- [observations]

### Per-View Summary
| View | Nav (ms) | Long Tasks | CLS | LCP (ms) | Commits | Scroll Commits | Bursts | Top Component |
|------|----------|-----------|-----|-----------|---------|----------------|--------|---------------|
| / | ... | ... | ... | ... | ... | ... | ... | ... |

Interpreting React Metrics

Signal	Likely cause	Fix direction
High commits, no long tasks	Frequent cheap rerenders	React.memo, stabilize props
High commits + long tasks	Expensive rerenders	Profile render cost, split components
High scroll commits	Scroll or intersection observer work triggering renders	Throttle handlers, memoize heavy list items
Render bursts (>5 in 100ms)	Cascading state updates	Batch updates, remove derived-state effects
react-scan: component with >30 renders	Missing memoization or unstable references	Stabilize props and parent renders
react-scan: component with >50ms time	Expensive render function	Split component, move work out of render

Element-source Follow-up

When react-scan identifies a rerender hotspot but you still need the exact file behind a concrete DOM node, hand off to $inspect-elements.

playwright-cli -s=prof-followup eval "async el => JSON.stringify(await window.__ELEMENT_SOURCE__.resolve(el))" e7

Use source.filePath as the direct edit target and stack to understand which parent components own the node.

Step 5: Cleanup

After profiling is complete and the report is delivered, verify no orphaned processes were left behind:

playwright-cli -s=prof-1 close 2>/dev/null
playwright-cli -s=prof-2 close 2>/dev/null
playwright-cli -s=prof-3 close 2>/dev/null

If the orchestrator started the dev server in Step 0, stop it now. If it was already running, leave it alone.

Notes

• Session isolation: each subagent uses a named playwright-cli session.
• Per-route collection: data resets on each goto, so collect before navigating away.
• Tracing: each subagent produces a trace.zip viewable in Trace Viewer.
• If __getReactScanReport returns null, the profiler still provides useful commit counts and render bursts.