GitHub's usage of history.pushState with a simultaneous lack of on-page DOM caching causes back/forward sequences to be painfully and epically slow as it destroys the browser's page cache (note: I don't mean the request cache); this is painfully evident if you go to a page like Homebrew's formula list[1], click to another page, and attempt go to back: everything locks up for a few seconds as the webpage painfully attempts to re-compose and re-render the massive document that a normal Web 1.0 era website would have been able to re-render nigh unto instantaneously as it would have been in the page cache.

[1] https://github.com/mxcl/homebrew/tree/master/Library/Formula