That's it, Michael! I never thought of increasing the meshing angle. That gave a lot of a boost! Funny, as I use MoI since version 1. But in my defense I have to say I never used MoI for such highly detailed and complex scenes I use it now. :-)

Anyways. I played a bit with the lighting schemes, too. In my case switching from metal shader to normal shader gave a noticeable speed boost, too. So, that's a also an option I want to consider.