Question is, whether one would really notice a slowdown of factor 2 in a browser. For example, if it takes some imaginary 2ms to close a tab, would one notice, if it now took 4ms? And for page rendering the bottleneck might be retrieving those pages.

2 - 4 ms? No. The problem is that many web applications are already extremely slow and bogged down in the browser. 500 ms - 1s? Yes, definitely people will notice. Although that only really applies to React applications that do too much, network latency isn't affected.

Yes, people will absolutely notice. There's plenty of interactions that take 500ms that will now take a second.