If you've ever tried to speed up a slow form, you've probably come across this link at MSDN. It is great and has some useful advice and troubleshooting.

However, I have found one single culprit that has caused an unbelievable performance hit in some of my forms, and recently talked to another designer who was able to fix a slow form by de-selecting a single checkbox in a dropdown control:

In both cases, de-selecting the checkbox to show only entries with unique display names quickly resolved performance issues. So, if you have a slow form, by all means, check out the suggestions on MSDN, but first, try changing your drop down controls and see if it makes a difference for you as well.