Tl;dr for anyone experiencing the same memory leak issues and looking for a fix: a possible solution is to disable workshop. It sucks not using it, but it made the plaza silky smooth for me at least 
So since 0.7 came out, people have reported of possible memory leaks in Lobby 3, but I believe that it isn’t limited to the plaza.
In my condo, some friends and I were messing around by trying many many workshop models over the course of around half an hour. The result was the exact same as in the plaza where my game was suddenly using up 13+ GB of RAM.
To test this, I disabled workshop and reloaded into the plaza. After sitting there for 5+ hours straight, the game was still running smoothly with ~2GB RAM usage.
Thus, it seems like the memory leaks aren’t really an issue with Lobby 3, but rather issues with workshop models that we didn’t really experience until now for some reason. @theuhie pointed out on the Discord that the game apparently loads the textures for workshop but never gets rid of these so maybe that’s the issue?
In any case, I don’t know how the game works and this is all just speculation on my part, but I hope that maybe this report can help the devs narrow down the issues, whether limited to the plaza or not 