The issues fidel describes can be encountered in certain conditions on a fresh N900, once you've set up an e-mail account and use the web browser.

Erratically, the device will spike CPU usage in some fashion. Unfortunately, it is not predictable and by the time the device is responsive enough to bring up top the process has calmed down.

So far the major culprits might be modest, its background daemon, telephony, or browserd. They will sometimes kill responsiveness entirely, other times they will just eat the CPU to the point that no software runs but the system still registers screen taps (you get a buzz, but no action) and the backlight switch can be turned on and off.