Your assumptions about Windows Mobile are correct. With time and use, the system does slow down and get bogged with junk/temp files. What many people use are clean-up type utilities such as
MemMaid or
SKTools . Doing hard resets every once in awhile and reinstalling apps by scratch every once in awhile is also a good idea.
For more info on maintenance, check out the tutorial called "Taking Care of your Axim." The link is in my signature. :)