3
High
Performance
Verified by measurement
Lab TBT 4,080ms on mobile
Lab Total Blocking Time is 4,080 ms on mobile, far exceeding the 200ms good threshold, freezing the main thread and making the page unresponsive during load.
⚠ Lab TBT 4,080ms on mobileThe page has a lab-measured Total Blocking Time of 4,080ms on mobile, which freezes the main thread and makes the page …
Both are photographs of the live page — the right side has the
change applied in the browser.
Evidence
Observed on the live site Verified by measurement
✓ Flagged independently by 1 of 7 analysts
- Lighthouse (mobile emulation, single synthetic run via DataForSEO): Total Blocking Time 4,080 ms against a ‘good’ threshold of 200ms. Lab data, not real-user field data — confirms the defect class, not the field percentile.
Business case
If this change wins
0 – 0
orders per 1,000 visitors
Medium
Effort — 2-5 days
Not A/B testable
At typical traffic
Detecting this needs about 315,206 visitors per variant. Ship it as a sequenced change and log the date instead.
Prioritisation
PXL score: 5 / 10
Assessed from the live page
| Question | Answer | Points |
|---|---|---|
| Is the change above the fold? | Yes | 1 |
| Is the change noticeable in under 5 seconds? | Yes | 2 |
| Does it add or remove an element? | Yes | 1 |
| Does it run on a high-traffic page? | No | 0 |
| Verified by direct technical measurement? | Yes | 1 |
Criteria that need your data
4 of these could not be answered from the live page. Session recordings, survey or support themes, and analytics access would raise the confidence of this ranking — they do not indicate a weaker finding.
| Question | Answer | Points |
|---|---|---|
| Discovered via user testing? | No | 0 |
| Discovered via qualitative feedback (survey, support)? | No | 0 |
| Supported by heatmaps or session recordings? | No | 0 |
| Found via digital analytics or real-user field data? | No | 0 |
Implementation
Hand this to your developer
- Surface
- Performance — PERFORMANCE
- Problem
- Lab Total Blocking Time is 4,080 ms on mobile, far exceeding the 200ms good threshold, freezing the main thread and making the page unresponsive during load.
- Current state
- notes: Total Blocking Time 4,080 ms (lab, mobile)
- Change
- notes: Total Blocking Time ≤ 200ms (good)
- Acceptance criteria
- GIVEN a first-time visitor on the affected page
WHEN the change is live
THEN Total Blocking Time ≤ 200ms (good), with no regression at 375px and 1280px widths. - Tracking
- event: cro_lab_tbt_4080ms_mobile on interaction with the changed element; verify it fires in BOTH variants before opening traffic.
- Success metric
- conversion rate on the affected step
- QA checklist
- Chrome / Safari / Firefox · 375px + 1280px · keyboard reachable · screen-reader name present · no CLS introduced
- Effort
- Medium — 2-5 days
- Test plan
- 315,206 visitors per variant (95% significance, 80% power); 37 days.
- Rollback
- Feature-flag the change; revert the flag if the primary metric drops for 3 consecutive days.