Mainstage 3 with Reason 7 as a slave CPU at Max

First make sure you don't run out of memory, if the system starts swapping there is no tweaking possible besides removing the memory load (mainly plugins with large sample libraries) or adding memory (an issue in a MBP).

If that is not the issue, you probably have a problem with the two audio engines (MainStage and Reason) running at the same time, fighting for the CPU. Maybe even some AudioUnits are in the mix (large Native Instruments plugins and/or Spectrasonics come to mind). That is a tough one, but MainStage allows you to tweak their engine via the Advanced Audio preferences. Pull back the number of cores used, safety buffer on. You have to experiment, the software sadly can't figure out what you want.

Also check if you can increase the buffer size, because that will help reducing the load and also add more stability.