Posting this here as I couldn't find a solution specifc for Universal Audio Apollo interfaces.
I actually figured out the issue.
Playback was stuttering in Premiere for any video I tried to view, no matter how small the file size, how small the resolution, the preview codec, using proxies, sequence settings, etc. This on a system I have been using for a couple of years with no issues. I don't know why I was convinced this was a video issue of some sort.
Somehow my audio preferences changed (without my permission or intervention) and were set to MME/ASIOVADPRO in Premiere. All I needed to do is change it to ASIO/Universal Audio Apollo.
Hope this helps someone else and saves them the time troubleshooting.
Hardware or GPU
User interface or workspaces