404 Errors on Admin Pages Following Update?

I don’t know if this is due to the new version of WordPress or what but more often than not a new site I am building returns 404 errors on admin pages when the pages don’t load fast. I suspect something in WordPress of returning 404 errors if the page doesn’t load within a few seconds. Alternatively it could be an issue with the default theme.

This is getting really annoying because I typically have to reload each admin page multiple times before it loads.