Having JS heavy web page will slow down the browsers. That even includes Chrome, you just need a larger number of tabs. On Firefox a few JS heavy site with a slow computer will make your site experience sucks.
Although not a single user would blame the site ( It is always the browsers' fault. ) but to me, most of the time, the less JS the better.
Discourse is pretty fast, but to say 300K zipped JS file is peanuts just doesn't cut it for me ( How about 100K? That is peanuts. ) And again, slow JS execution and loading is something i have zero control of.