The amount of stuff on the iPod should have little to no effect on the speed of the menus, etc. I have almost 90GB of music on my 160, and it works virtually the same today as it did two years ago.
You can try resetting it (equivalent to rebooting it) by pressing the Menu+center buttons together for about six-seconds.
If the reset does not alleviate the problem, you can try restoring it (more drastic as
this will erase EVERYTHING from the iPod). You can
Click Here for instructions on restoring.
Before you run a restore, make certain that you have all of your music, videos, etc, available on your PC before begin since you will need to resync everything after the restore completes.