I assume you have already switched the device off, and done a fresh cold boot. If that didn't resolve it, then you'll need to try to identify what is going on when it is getting sluggish.

I'd suggest installing "osso-statusbar-cpu". It puts an icon in the tray that shows CPU load and available memory, and if you tap on it you can "Show processes" to see the top resource hogs. You can also get the same kind of information (but no pretty graphics) by running "top" in the X-Terminal window.