Torrented FINAL CUT FTW!!!! I use mine as well and it runs fine. SO here is the break down
MBP: What are your specs, that would really help. Anything with core 2 duo will run well.
Bottleneck performance: Make sure you use external disk for scratch space. You can use a dedicated flash drive which is 8 GBS+ if your working on small stuff, but i usually work with 20+ min plus footage, so for good performance you will need external hard drive there is no doubt in that.
Think of it this way, there is two streams, one going up and one going down, and they work well to run your laptop and software, but as soon as you start using something such as final cut that constantly flips gigs of data to the hard drive and ram, your laptop will work unusually low.
You also need 20% free space on the hard drive in order for it to work efficiently.
if you got all the above, reinstall your laptop. I usually do mine minimum of once every 7-8 months, but usually i like to have it nice and clean for every school semester.