Just want to report I am experiencing frame drops on the new Roku Ultra. I am seeing this in Plex with 1080p bluray mkv remux's. It is especially noticeable on scenes with action or panning. The same files play perfectly on my Roku Ultra 4670X.
Let me know if there are any troubleshooting steps you want me to perform or logs I can pull.
Again, you need to report this directly to Roku per the details I stated above (the more people that report this, and sooner, the more likely and sooner it gets fixed).
Also, since the issue involves audio, make sure you have the 4800 set to "Auto passthrough" and not "Auto detect" mode ("Auto detect" transcodes/converts all audio to the highest Dolby level of the connected device).