Hi,
There is no simple answer but in general I would say that your computer is not running efficiently or is not up to the task. There are a number of simple things you can do to give you computer a hand ...
1. shutdown any programs you dont need running
2. don't run virus scan or defrag whilst editing
3. before starting editing defragment your hard drive and do it frequently
4. disconnect from the internet while editing - this will stop auto updates
5. edit shorter videos and then add them together later
6. do your rough-cut edits first without any fancy transitions or effects ..add later
7. on a laptop make sure you are on mains and set to high performance
8. make sure you have plenty of space on your harddrive - if necessary get rid of stuff you don't access regularly or back-up to external media or if possible fit a larger capacity & faster spinning harddrive
9. don't add videos to the timeline from dvd's or memory sticks - move them to your hard drive and load them from there
10. avoid re-editing already edited and exported footage

If you are trying to edit AVCHD footage try converting it before using MEP

hope that helps