I am having this exact same issue, even on the latest stable 7.1.1. It's a downgrade from my A310 GPU, or my old 12th gen i5. Did you ever figure it out? I can't get it to do more than 1 4k to 1080p transcode without hiccups.
Update: I just updated my motherboard bios to the latest version, it was on the original first bios, and now everything is fixed. I just got up to 6 4k to 1080p transcodes going with no slow down in sight.