Usernames and passwords are kept locally on your computer. If it doesn't display it when replying to messages for instance, your cookies might be messed up. There's a maximum number of cookies that can be set by your browser (I believe it's 256). And nowadays, just about every single website sets a cookie on your system (most of the time it's a banner on a site that sets the cookie).
Cookies are set on a firts in, first out base. So if you visit a lot of different pages, the cookies for this site will eventually get lost.
As for the slowdown on sunday: I think the server was doing some administrative tasks (cleaning up log files and such), it does that every now and then. There's more than enough bandwidth. Or it could be that the trans-atlantic connection was slow due to some router error somewhere...