Agree on the horribly slow bit. (can't quite comments on how it compares to github, but this is the most recent issue I found dealing with the site's slowness)
With FF22 Nightly, switching to a pull request with a lot of changes (several hundred lines) can cause the entire browser to lock up for 10-30 seconds while it tries to load everything. Once loaded, scrolling is smooth.
With IE9, it loads the comments quickly and doesn't seem to hang while loading the commits, but once the commits are loaded, scrolling is jerky and unresponsive.