IMac – Using an iMac as external monitor for the mid 2009 13” MacBook Pro

Here's a few things I would try:

1. Do basic system maintenance (repair permissions, clean caches, run daily-weekly-monthly maintenance scripts). I like ONYX (http://bit.ly/eCzYfj) for doing this, but there are plenty of other options that do the same thing.

2. Make sure you have at least 20% free disk space. If not, try clean up and free some space.

3. If things are still slow, try creating a new user account and logging in with that. If that account runs much better you still have some cleanup to do.

4. Check your login items (in system preferences, User & Groups). If there are login items you don't recognize/need try disabling them.

I would also look into investing in at least a 2MB memory upgrade. Not too expensive and it looks like you could use it.

Good luck...

There's an FAQ on the Apple Support page: http://support.apple.com/kb/HT5219?viewlocale=en_US&locale=en_US#4 Since your Mac is Mid 2010, it does not have a thunderbolt port.