Twitter has the autofocus javascript snippet for login at the bottom of it’s page code, understandably so for performance optimisation. On the other hand, this leads to poor usability because of user frustration.

This is because if the page finishes loading after users have started entering in their passwords, the focus will move to the username field because of the autofocus javascript snipper. When this happens, unless the user looks back at the screen, this will reveal the password they are entering in partially or completely. 

It is not only performance vs usability here, its not performance vs usability and security, and thus this is something that I feel Twitter should and will move up the page code.