Really hoping someone here could give me some ideas because I'm all out. I've noticed in CC2020 (and now confirmed in 2021 as well) that playback in timeline stutters and drops frames. Playback in my Source Monitor window is FINE, it's only when I add a clip to a sequence that this occurs. Now what's weird is after experimenting I realize that it ONLY occurs when a clip is loaded in the Source Monitor...if I close the clip or toggle to the effects pane my timeline in Program Monitor plays normally. So in other words, when there is a video clip loaded in BOTH my source window AND Program window, the playback in my Program window and timeline is choppy and dropping frames.
This occurs whether I'm editing 4K, 1080p, using proxies, lowering the playback resolution, removing effects, etc. It's an issue unique to these two versions as when I switch over to CC2019 (or earlier) it does not occur. Feels like I've tried everything suggested in these forums (including turning off default audio input, resetting preferences, trashing caches, etc.) My setup/source files/system disks in PP2019 are all the same, the only difference seems to be the software version. Any ideas welcome as I really would like to use the more recent versions of Premiere, instead I'm relying on PP2019 for client work!
iMac 5K Retina (Late 2015), OS Mojave
Processor: 4GHZ Intel Core i7
Graphics: AMD Radeon
Output: Blackmagic UltraStudio Mini Monitor (connected via thunderbolt)
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Try removing the BM monitor from the playback chain and in prefs. If that solves your problem, then the BM hardware/software could be at fault.
Something else to consider: macOS Mojave vs macOS Big Sur vs new editions of Pr that are optimized/friendly for/to Apple silicon.
Thank you, Jeff! Sharing in case others have this issue: Although this wasn't the solution it did compel me to take a closer look at the Blackmagic Video Setup (which needed an update to v12.2) and the Premiere/ Preferences/Playback settings themselves... the update itself did not fix it, but what did was unchecking the box " Disable Video Output when in the background" from the Premiere Playback settings. The Blackmagic device itself has its own settings for turning the video on/off when in the background so maybe they were in conflict with one another? This was also occurring before when toggling between 4K footage which the device scales down for output and 1080p which it displays natively. Either way, your zeroing in on the BM device helped me solve it so much appreciation!
UPDATE: Also seems to help if I change Source Monitor settings to a lower "Paused Quality" from within the window's settings...no idea why the three of these (Source & Program windows in PP and External Monitor via BM) would have this particular combined effect of dropping frames, but there it is.