Multiple Instance performance problems

I am having trouble with multiple instances of RTAS on Mac with my plugin.
All is well on VST, AU, Windows RTAS. The performance is poor once a second instance is added.
I noticed the way the audio thread is handled is very strange. As I was stepping in the debugger, I was the GUI meters drawing.
I find that odd that the audio thread would yield to the GUI.

Anyone have advice as to how ProTools manages its audio processing threads vs GUI ?