Well, the thing is, performance in DCS is usually good.
Like I said, I play often on our squad server, using a very complex multiplayer mission with lots of units and scripts, and I get between 50 to 80 fps now that I tuned everything right (with all the options maxed out).
Only in these two missions, which have in common that they include these additional models, is performance so bad.
I think I'll try and add some of these models to my squad training mission, just to see if performance plummets or not.