I have no experience with that.
I'm running Lr 4.3 and it's not slower than Lr3.
I remember some people have said they would want to switch back to Lr3, but don't know if that helped.
Have you read through this humongous thread: http://forums.adobe.com/thread/971581?start=0&tstart=0
It has over 300 posts - all about speed.
And here is another one saying that Lr 3 is slow: http://forums.adobe.com/thread/656635?start=0&tstart=0
How many hard drives have you got?
The bottle neck with today's computers is the reading and writing to and from the hard drive. If there's only one drive in you book, that's probably where the bottle neck is.
Keep in mind that Lr continuously reads from and writes to the catalog. When you are in the Develop Module, each step is written to the catalog and immediately it and all the previous steps are read back. As an aside: That is probably the reason why clearing the History helps performance, particularly with the Clone Tool.