# TBT 1,500ms on weather — dev spec
Site: msn.com · Priority 6 · High · Effort: Medium (2-5 days)

## Problem
Lab TBT on /en-us/weather is 1,500ms, blocking main thread interactivity and causing unresponsive taps during load.

## Evidence (from the live site)
> Lighthouse (mobile emulation, single synthetic run via DataForSEO): Total Blocking Time 1,500 ms against a ‘good’ threshold of 200ms. Lab data, not real-user field data — confirms the defect class, not the field percentile.

## Current state
notes: Total Blocking Time 1,500 ms (lab, mobile on /en-us/weather)

## Required 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)
- AND no layout regression at 375px / 768px / 1280px
- AND the changed control is keyboard-reachable with an accessible name

## Tracking
- Fire `cro_tbt_1500ms_on_weather` on interaction with the changed element
- Verify the event fires in BOTH variants before opening traffic

## Test plan
- Two-proportion z-test, 95% significance, 80% power
- 315,206 visitors per variant to detect a 5.0% relative lift
- Run at least one full business cycle; 16 days

## Rollback
Feature-flag the change; revert if the primary metric drops for 3 consecutive days.