2023, encountering the same bug here on Davinci 18.1.4. One way around it for me is to
disconnect the second display,
then launch Davinci fully into the project,
then reconnect the second display. Only a partial solution, as the dual monitor option is greyed out this time, but that's okay for me as I simply put the waveform or other utilities on the second display. Given Davinci performs like a champ if it "thinks" that there is not second monitor running and using resources, the evidence then is that the problem is within Davinci and not the system or peripherals.
UPDATE -- of note and 'tis weird: The smooth playback only happens on the tab that's active when reconnecting the monitor after launch. For instance, if I'm in the color tab, its viewer will be smooth, but the edit and fairlight viewers not --
even if I [i]return to the color tab later in the same session (it still is smooth)[/i]. (This behavior is ever more like a bug, right?)
So again, the only way I can get Davinci to perform correctly while a second monitor is connected is to launch Davinci with the monitor temporarily disconnected during launch.....
FYI & Thank you!
iMac Pro Monterey | 2x Radion Vega 16+8 | 64gb | 16 core