To get a handle on performance in Resolve, you need to be aware of what resolution is used for processing. This depends on where you are: In the color page? Using OpenFX and Fusion Effects added to the timeline? Creating a Fusion composition from the timeline?
Most work in Resolve happens at the chosen timeline resolution. However, when taking individual clips to Fusion you are working on the source resolution – unless you applied super scaling!
Proxy settings in Resolve help temporarily reduce the processing resolution. However, in Fusion an additional proxy setting, as well as an Auto proxy, is available. Check out the video for a deep dive.