I can't tell you how it compares to the last Firefox version since I didn't use that, but I haven't noticed any slowdowns yet and I've been using it all day every day since I wrote this post, so four days now. 0.01% CPU usage when it's in the background, 722 MB physical RAM with two tabs open right now. It might also depend on what OS you are using, I'm on macOS High Sierra, your mileage might vary on Windows.