Yes, thanks for the advice.
fwiw, I've given VTune a initial run and it's shown some anolmaly, in that when the C::B compiler plugin is enabled the CPU is idle for 70% of the time between launch and the GUI opening, so I'm not sure what that means if the problem is CPU bound.
I'll need a run quite a few variants on C::B launch to see if I can make sense of this, and if I find anything solid I'll report.