Hey everyone,
I've been having this problem the last few days too, on a WINDOWS machine. Complete drop in performance with no explanation, originally in V.17 so I upgraded to 18.5 and still had the same issue.
Main symptoms were that in the cut and edit page the playback was lagging despite the FPS ball being green and reading 25fps.
I tried all manner of things; lowering timeline resolution, creating optimised media, creating various proxies but it just did nothing, toggling on/off audio, linking/unlinking clips, disabling grades.
It seems I've finally figured out a solution!
As I mentioned, I'm currently Windows-based.
Step. 1 Right-Click DaVinci Resolve icon on the taskbar.
Step. 2 In the pop-up window that appears Right-Click on the new DaVinci Resolve icon that is displayed
Step. 3 Click "Properties"
Step. 4 Click the header "Compatibility"
Step. 5 In the "Compatibility mode" section uncheck the box for "Run this program in compatibility mode for"
I restarted DaVinci and performance levels returned to normal.
Credit to this Video for suggesting the method -
https://www. youtube.com/watch?v=bFVywDATYVk&t=249s
Let me know if this works for others.
Best
Lx