1. What version of OS X are you using?
2. What version of iMovie?
3. Does the MBP only run slowly when you are using iMovie?
4. What are you doing in iMovie when it runs slow?
5. How much RAM do you have in the MBP?
6. How much free space is left on the hard drive?
I use a macbook pro running very slowly, do not know if it's related or not.