Reasons Why It Happens
1. You've taken up all of the space on your phones internal memory. Just like on a computer using all of your systems internal hard drive memory will leave your device running awful slow. I suggest installing applications directly onto your memory card and not on your phones hard drive to speed things up.
2. You simply have too many applications open at one time. On the Windows Mobile start button I would suggest checking out the task manager and making sure apps your not using are closed. Ultimately make sure to exit your apps when you're done with them. Remember phones are still lacking in terms of RAM which can lead to slow systems.
3. You have too many hardware options running at one time. These days devices come with GPS receivers, Bluetooth, FM Tuners, Waffle Irons, okay maybe not the last one, but you get the point. Hardware options like apps should be closed when not in use, this will help easy processor tension and thus speed up your system.
4. Viruses may be the culprit. Scan your device for viruses using a program such as Kaspersky. Remember, if its Microsoft made it's going to get viruses and Windows Mobile is no different. If you began realizing your slowdown after surfing the web from your device or downloading an unrecognized file this may be your slowdown mechanism. Remove the files after using a virus scan option and your device should speed up quickly.
5. Your memory card may be corrupt or nearing full capacity. If you tend to run applications off of your memory cards they need to be routinely examined. For this task I suggest Pocket Mechanic Professional, a program that can defrag your cards, find errors such as bad sectors and in the most simplest of terms, make your memory cards work as if they were brand new.
Those are just 5 reasons for system slowdowns that I have recognized on a rather regular basis. Simple close all of your hardware and software offerings when not in use, and have a third party application scan your memory cards for errors once every one to two weeks and you should realize faster phone speeds in no time at all.