IMO, complex scenes are one of P5's biggest downfalls (when it works), it just can't handle them well, if at all. If you have P4 reinstall it and it should work much better. If you're going to do complex scenes often and don't have P4, you may want to consider a "downgrade." I know it sounds silly, but P4 (preferably with ProPack) is the way to go for this type of scene; working with P5 is like working in mud compared to it.