To explain this a little: (I'm a choob though so correct me if I'm wrong please)
Websites run off bandwidth. They get a set amount of user storage and through-space to use. If they overuse this, it bogs down the connection (this is how DDoS'ing works by the way).
At certain times of the day, the site runs more smoothly because of timezones where people go to sleep (which is why as I post this it is almost 1am and the site is running very nicely). However, as soon as everyone goes back on the site, it bogs it down and that's how it becomes slow.
The remedy to this is to increase server bandwidth or upgrade servers entirely.
