I had the problem for some time that Lightroom, or more specifically its process dynamiclinkmediaserver, took 400% of the CPU (that is, the 4 cores available). Even after waiting several hours, that wouldn’t change, it seemed to be in an endless loop of some kind. Killing the process didn’t help, Google didn’t either (hence this post).
I created a new catalog and imported the old one. Since then the problem seems to be gone. You should try this if it happens to you. And please post your experience here as a comment to build up the knowledge on this issue!