as a saas founder, I can confirm you are 100% correct about this and we'll be looking at IronCalc for https://www.definite.app/
It was the only page open in Chrome, freshly launched specifically for this test. Safari has a bajillion open windows and tabs. Something about how Chrome handles the page is definitely odd.
Just for fun, I also tested in Firefox, which was already open with fifteen windows, most of which have a handful of tabs. No lag there.
This has been tricky to reproduce.
I did load up Chromium to check it and got the lag there as well: 124.0.6359.0 (Developer Build)
I don't use Chrome almost ever. Maybe my outdated version is why you can't repro.