Several Lightroom users have reported poor performance in different modules while using Lightroom 4.
Aside from working internally to resolve as many of the causes we can via Lightroom updates, we have put together a list of several suggestions to try in order to improve the performance of Lightroom on your computer and in your workflow.
If you are not seeing the performance you would like with Lightroom 4, have a look through these suggestions to see if they can improve the speed you're getting out of Lightroom.
Lightroom Performance Hints: http://adobe.ly/LRPerformHints
If after working through those suggestions you are still not up to adequate speed feel free to join other community members on our forums for further discussion: