Play in Edit mode is very fast.

I'm in Resolve 17.4.2 Build 9 and just reopened a project that had been finished in Resolve 16. Its only 44 sec but when I played it in the Edit page it zoomed by in less than 10 sec . If I switch to the output page it played properly . What's going on ?

turned computer off and on - fixed- duh

This can usually happen, when Resolve does not find the SDI out or has other issues with the video card. A common issue is that there is a different software running that already uses the video card. That's why restarting the computer "fixed" it. Usually it helps to close other software and maybe change the Monitoring Settings to something else, hit save and change it back to the desired settings, in order to inform Resolve "hey this is what you should do. Pay attention"