I noticed this on RC2 and I just installed RC3 and get the same issue. Downloading headers with compression spikes CPU 75% and higher resulting in sporadic mouse cursor freezes.
I thought it might be Giganews Accelerator. I took GNA out of the loop and got the same issue. It goes away when header download is complete but during header download the real throughput can vary wildly. Sometimes it can take a couple of minutes to get headers. During that time its move the mouse then watch and wait until the mouse cursor moves.
Other than that things seem solid so far.