All of them, to some degree or other.
Okay. I don't want to buy a new computer but I suppose I could replace the video card (now: AMD FirePro V4900) or install more memory (now: 24 GB). Would it make sense to do either of these things and if I was going to do only one, which one should it be?
What Version of InDesign?
How many RAM do you have? How much could you add?
How much space on disk is free?
You use tools to look at what is going on. For instance, how much CPU is being used by Photoshop? How much memory?
These things are EXPECTED to be slow. That's why you can turn them off/.
Upgrading the video card makes more sense than adding more RAM, but I doubt it will make enough difference to justify the expense. Consider setting the Display Performance to Allow Object Level Display and set these monsters to Typical instead of High Quality, and or make sure Live Screen Drawing is set to Delayed or Never, and/or turn off page thumbnails in the Pages panel, any of which should speed rendering. Turning off Live Preflight might help, too.