I've had this intermittent issue with poor performance when watching 1080p50 or 1080p60 video (internet streaming e.g. YouTube) on my RPi 3, which seems to have got consistently worse since upgrading to 9.2.3 and seems to happen all the time now.
The problem I see is a lot of dropped frames, or stuttering.
If I try MMAL instead of OMXPlayer, I get no stuttering (audio plays fine) but a massive amount of dropped frames for prolonged periods. Either option is mostly unwatchable.
Locally played videos seem fine, though I don't have any 50/60 fps content. 1080i50 from TV broadcasts play back fine (tvheadend live or recordings). I've not encountered any issues with 30 fps or lower framerates.
I increased the memorysize to 52428800 in advancedsettings.xml as I was seeing some buffering issues in the logs previously, but this issue persists.