Mainstage lags/freezes when opening patches containing 2 external instruments output to IAC channels

OS 12.0.1 Mainstage 3.5.3

Driving me nuts this as it's making Mainstage effectively unusable. Opening a patch containing more than a single external channel routed to IAC causes a graphics, sound, midi freeze. Even the cpu monitor stops. Any thoughts?

Have spent several frustrating hours with apple support, none of whom knew what the IAC devices were, which added to the frustration. So I've done the deleting prefs, removing plugins etc