hey, thought this bit of tech trouble shooting might help here.
I was having issues w gpu simulation rendering in c4d. The issue seemed to come down to easyres. Once i quit it, things worked as expected.
The external display was unable to properly use gpu rendering of simulation in windowed c4d unless the laptop was closed or strangely qt was screen recording. So easyres seemed to be doing something wonky w gpu rending priority or screen resolution adjustment. guessing this could also cause slow downs / issues with vdmx.
edit, - there might be more to it than this app. might be os x preferring certain screen resolutions / combos.