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.
