3
High
Performance
Verified by measurement
Lab TBT of 1,920 ms on mobile
Lab TBT of 1,920 ms on mobile makes the page unresponsive, delaying interactions.
⚠ Lab TBT of 1,920 ms on mobileLab TBT of 1,920 ms on mobile blocks the main thread, making the page unresponsive and delaying interactions.
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 1,920 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 TBT of 1,920 ms on mobile makes the page unresponsive, delaying interactions.
- Current state
- notes: Total Blocking Time 1,920 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_1920_ms_on_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); not testable at current traffic.
- Rollback
- Feature-flag the change; revert the flag if the primary metric drops for 3 consecutive days.