> an embarrassingly large amount of the software I use on a daily basis for productivity runs in a single thread. My expectation was that UI blocking would be rare- it isn't, particularly with Chrome, Firefox, and Slack. Jira in the browser is terrible- even with insane resources and 1Gbps bandwidth I regularly have to wait 10-15 seconds to be able to enter text.

Why would you expect anything different when JS _does_ run in a single thread? We'll have to wait for WebAssembly to have anything like real multithreading, with good-enough performance, on the Web.