Page speed test API: read Lighthouse performance results

Run a mobile test

Set STRALE_API_KEY to your API key, then run:

curl -X POST https://api.strale.io/v1/do \
  -H "Authorization: Bearer $STRALE_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "capability_slug": "page-speed-test",
    "inputs": { "url": "https://web.dev/", "strategy": "mobile" },
    "max_price_cents": 5
  }'

Use a public URL that Lighthouse can load without signing in. strategy accepts mobile or desktop and defaults to mobile. Keep it the same when comparing results before and after a change. If the page cannot be loaded, handle the error separately from a low performance score.

Read the measurements

In output, performance_score runs from 0 to 100. Strale assigns grade from that score: A at 90 or above, B at 70–89, C at 50–69, D at 30–49 and F below 30. The letter is Strale’s grouping, not a separate measurement.

• lcp_ms: Largest Contentful Paint, in milliseconds.
• fcp_ms: First Contentful Paint, in milliseconds.
• tbt_ms: Total Blocking Time during the lab test, in milliseconds.
• ttfb_ms: server response time, in milliseconds.
• speed_index: Speed Index, also in milliseconds despite the field name.
• cls_score: Cumulative Layout Shift, a score with no time unit.

For example, lcp_ms: 2400 means 2.4 seconds. Individual values can be null when Lighthouse does not supply a measurement. Read null as unavailable, not zero.

Keep lab and field data separate

Google distinguishes lab tests from field data. This Strale tool returns Lighthouse lab results; it does not return CrUX real-user data, INP or a Core Web Vitals pass/fail assessment. TBT is not an INP measurement. Avoid presenting a lab result as proof of every visitor’s experience.

Choose the next improvement

Read opportunities for suggested changes and estimated savings, and diagnostics for supporting findings. Estimates can overlap, so do not sum them or promise the stated saving. Make one change, repeat tests with the same strategy and compare the trend. Scheduling those tests is your application’s responsibility.

See Page Speed Test for the complete inputs and limitations. Header Security Check is a separate check for HTTP security headers; it does not measure page speed.

Tools in this guide