You might be referring to the Spectre mitigation changes:

Timer precision from performance.now and other sources is reduced to 1ms (r226495)

https://webkit.org/blog/8048/what-spectre-and-meltdown-mean-...

https://trac.webkit.org/changeset/226495/webkit

Although you can claw that precision back by enabling cross-origin isolation for your site, at least in Firefox and Chrome, which both quantize high res timers to 100μs in non-isolated contexts but only 5μs in isolated contexts. I'm not sure exactly what Safari does.