Then it might indeed be the case that this machine isn't able to cope with the load, sorry to say so. :-/
Usual workaraounds encompass "freezing" a few tracks, or using Bus/Group tracks with one common instance of MIR for several instruments routed to this bus - ideally ensembles of the same or similar instruments, eg. "1st Violins", or "Low Brass".
I don't know the interface you're using, but it might also be worth trying different hardware with different drivers, just to see if it changes anything.
HTH,
/Dietz - Vienna Symphonic Library