Several days ago, my BlackBerry became so slow after I have installed a new theme. It became so slow that even several resets failed to fix whatever the problem was that I was forced to perform a full security wipe. I initially tried a partial security wipe which involves deleting only the applications while retaining my contacts and email as well as my files saved in my media card.
If for some reason your BlackBerry is lagging too slow or loads too slow after you have installed several applications, and deletion of that application did not fix the problem, I usually would just go and reset my BlackBerry to factory default. This is not really hard and RIM makes it easier for you to reset and reinstall your desired applications easily. To do a reset, simply go to Options> Security> Security Wipe. You will be asked to type blackberry to confirm. No sweat.