My server is suddenly running very slowly, why?

[Rob Swan]

This might sound strange, but Ubuntu sometimes slows down if it needs updates, I would first follow the update procedure in THIS POST.

If that does not fix it, I would then follow the procedure in THIS POST to give it a bit of a clean.

If this fails, reboot your router.

The only other thing is if you have port 22 for SSH open on your router, I would close it and then reboot your server.

If it is still running slowly, you could be looking at a hardware failure on your server, but before you assume that your server has a hardware fault, I would try a full reinstall.