Yeah, I get it. But the bottom line is you're using a version of Sonar that doesn't have 100% multi-core-support (If it has it at all). That's going to be a problem with most recent VSTs. Also, Sonar 8 is way more optimized, so will give you better performance on an older machine.
Speaking as someone who started Sonar on v2, then had 6, 8, and all the X-series, v8 is one of the more solid older versions (and what I would use if I were in your position).
The reality is, Sonar 5 is fine if you are content using older VST plugs. But once you start thinking of adding newer VSTs, especially a resource hog like Superior Drummer 2, then you really have to re-think your setup. IMO Sonar 8 is the best compromise (least painful and most beneficial way to go).