It’s a combination of various factors. Personally, I like that it ensures that recently played stuff gets downloaded, because I am more likely to listen to recently played stuff again rather than, e.g., “most” played stuff from 3 years ago.
I think part of your issue is that you just enabled it. I have it enabled all the time, my download space is always full, and the stuff it downloads is quite reasonable. And I can still choose other things manually. It’s not very slow for me either if ARC is in the foreground (it is slow in the background but I am on iOS where this is expected and it does not bother me because it can do its thing whenever)