Had the same problem man. Pissed me off so bad but I fixed it.
1) close any background operations
2) boost your systems ram memory with a flash drive. if you dont know how to do this youtube it really simple.
3) I think this is what really did the trick for me. If you are running Windows 7 or vista to both do a lot of crap to make your computer look pretty that you don't really need. Right click my computer.. Go to the properties. Find advanced system settings. Go to the "Advanced" tab within advanced system settings. Select performance and then optimize your system for best performance. Your computer will look like a windows 90 machine but it'll be able to render your vid hopefully