we had something similar last week on windows 7
fix was to right click desktop>personalise>change to windows 7 basic mode
basically it turns off all the fancy windows 7 transparency
as suggested for avid setup
it worked and the frame rate came back to normal in Resolve
I was convinced it wasnt the graphics cards as it was something conflicting with the blackmagic card being on as an output (i turned off the external out and it went full frame rate)
try that