Is it taxing on the mac computers' performance to have multiple user accounts and not log out of them?
macOS is a multi-user system so any application opened by any user stays open til it gets closed. You can have a look at Activity Monitory to see that even with just one user logged in there is a lot of additional stuff going on in the background. A lot of these processes usually sleep/wait for any input, the same is valid for user applications which often are sleeping (with their memory swapped to disk) if they aren't actively used.
If you have a set of applications (i.e. M, N, O, X, Y, Z) you regularly use it doesn't make a significant difference on memory or CPU needs whether you use all of them within the same account or use Fast User Switching to switch between account A (running applications X,Y, Z) and account B (running M, N, O).