For quite a while I was having problems with Omnisphere running smoothly in Sonar. After a lot of experimenting, I've narrowed things down to this:

Omnisphere runs fine until I load a "MIDI learn template" I setup for Omnisphere. I use a M-Audio KS-88 Pro Controller, and have every knob, slider & button assigned to control something in Omnisphere. As soon as I load that template into Omnisphere, Sonar's GUI becomes very sluggish. Specifically if I select a clip in Sonar's track editor it'll take a few seconds to react. As soon as I unload the controller assingments from Omnisphere, Sonar goes back to normal, and selecting a clip is instantaneous.

I have X-ray disabled in Sonar & keep Omnisphere's GUI closed when not editing or loading a patch. I've experimented with different sound card buffer settings, different latencies, different I/O buffer settings in Sonar & nothing helps. The issue was also identical in Sonar 7 PE, 8 & 8.3.1 PE. Any ideas (short of not using controllers to control Omnisphere, which would be a total bummer!)