Okay, I’ve done some testing and I think I’ve figured it out. Basically, the old indirect illumination from physical render was relatively fast because it was using a low sample count — much lower than the “low” setting in the QMC default.
I find I can get a slightly better (less grainy) result a little faster using the following settings:
Primary method: QMC
Secondary method: Radiosity maps
Custom sample count: 24
Radiosity map sample density: 50%
Everything else at default.
Give it a shot and let me know what you find.