I am not so sure it is the pre-rendering at all.
When I play live, i.e. keep a fat chord pressed without recording and playing back anything, the load is also clearly higher in Mulab.
Here the same Repro-5 pad, 5 continuous notes in HQ mode, with MC support on (without Repro-5's MC support, Mulab is in serious trouble, so there is no point), on the left Mulab, on the right Reaper:
I have no idea where that huge difference comes from...