You should first figure out if they’re just running slow because of your machine. If you have an older GPU you can adjust rendering modes to see if one performs better than the other by hitting Ctrl (Cmd)+E. GPU acceleration is typically fastest.
If it’s very complex and not just because someone was adding a lot of surplus points, and your machine is up to snuff, you can try moving objects you don’t need to layers and hiding them. Hiding objects should improve your rendering speed.
Lastly, you can check how much memory your system is actually using by either using Windows task manager or OSX Activity monitor. I’ve got some files I would consider very complex and even those only end up using a few gigs of ram at a time. FWIW I always found Illustrator performance better on OSX, used to always have tons of stability issues on my PC.