Fusion in Resolve 15 SLOW vs Fusion 9

Had a simple shot. Track the shot, create a 3D scene and add one image onto an image plane in the scene. In the new Resolve Fusion 15, every step of the process was tedious. Buttons were slow, turning things off/on was slow. The solve was slow, track was slow. And once the shot was done, I could not get it rendered out! I tried the disk cache, I tried the RAM preview but it ran out of RAM (I have 64GB system memory and 11GB VRAM), and even from the edit page, it took several times playing it to get it to chug through and cache it until the playbar was blue. And after that, it wouldnt play back faster than about 5 or 7fps. Couldnt play it back in fusion either. So I switched to Fusion 9, re-did the shot and everything was snappy. Things turned off/on instantly, and I could even connect and disconnect nodes during playback without interferring with the speed much. Without rendering it played back at about 15fps. It took just a few mins to cache up and it played back in real time. So my question is this -- why such a big difference in performance between Resolve 15's fusion and Fusion 9?