Resolve 15 beta performing very slow

I completed a simple A over B composite in Fusion in Resolve. And for some reason, Resolve is now extremely slow since I reopened the project. It's a 53-second long shot. But, its just a greenscreen over a still image background. No fancy effects, except a Grid Warp.

Any thoughts as to why its slow, and what can I do to improve performance? I'm on an iMac Pro with 32GB of RAM.

Thanks in advance.