I've not noticed a speed loss due to multiple undoes.  I do know that IBL and ambience occlusion take the longest to render, especially with a large image size.  If you are rendering in a separate process it definitely helps to make your bucket size smaller.  I have a Core 2 Duo processor with an intel mortherboard and 2 gigs of Ram memory.   While e-frontier reccomends increasing the number of threads to 4, I've found and other users as well with a Dual Core that using 2 threads actually is faster.