Slowness and lagging is usually attributed to Memory (or lack there of). Menu>Options>Status (What's your free memory?)
If it's less at 12 to 15mg you need to do some house-keeping.
To free up memory: How do I free up memory on my device? - BlackBerryFAQ
Also third party applications and non-vendor themes take up a lot of space. Start by deleting the sample video and unused languages. If you don't have a media card get one. Get all your pics and music of the device memory and onto the media card.
Be sure that you are closing your programs properly if you don't they will be running in the background eating memory.
To see what's running ALT + Escape. If there are more than the base 5 programs you have things running in the background.