Putting on eng manager hat, the problem to solve is that this regression went undetected, not that Safari is slow.
The solution is a test that fails when Chrome and Safari have substantially different render times.
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That test will be disabled for being flaky in under a week because the CI runners have contention with other jobs, causing them to randomly be slower and flake, and the frontend team does not want to waste time investigating flakes.
"Just have dedicated runners with guaranteed CPU performance", but that's the CI platform team's issue, the frontend and testing teams can't fix it, and the CI infra team won't prioritize it for a minimum of 5 years.