Brief followup: I have been talking to Apple support about this. They have suggested deleting and re-downloading all the books in my library, restarting in Safe Mode, and finally even went as far as to say I need to reinstall Monterey (not too painful nowadays, just the operating system). I have done all these and it is still slow and laggy. I am in the middle of continuing to troubleshoot. I suspect that the old highlights I've added are somehow confusing Books or making it slow to render. Whatever the problem, it's very hard to use the way it is and I'm surprised there isn't a deluge of complaints yet. I'm dying to know what is causing this.