While in Rewire mode, Reason does not show how much CPU resources it is using. Sonar only shows what it is using independent of Reason. So together, they could be chewing up more CPU horsepower than you can see. Try raising the buffer size in Sonar Option - Audio - Buffer Size / Latency. This can not only lower the CPU usage, but can smooth out the performance. Try loading the Reason simple default song and then a complex demo and see if the performance is affected. If so, you may need to do some other things to tweak your PC for audio, such as searching in the knowledge database for PC Optimization, and follow directions to tweak Windows for audio.