[Solved] Pi4 4GB mythtv-light 31 frontend only HD performance issues

[SytheticGate]

edited with solution:
of course I walk away for a bit and then solve it. It turned out to be a bad network cable between the backend and the switch, so the backend could only do ~10Mbps of network traffic to the outside world. Replaced the backend cable and the pi 4 is working great!.

Original post:

I spent a good part of yesterday trying to figure out what is going on with this thing, but no luck so far.
This is a fresh install of rasbian, I followed the instructions at the raspberry pi section of the wiki but am unable to get smooth HD video going.

I've done the pi firmware updates, am using the FKMS driver, using the V4L2 codecs etc. In the Setup Wizard, the SD test plays smoothly, but the HD test stutters and glitches the audio. I turned on the playback data overlay and I can see the storage to buffer rate (~9.8Mb) lower than buffer to decoder (also ~9-10Mb) and the 32Mb buffer runs empty. Frontend logs show "Waited XXXms for video buffers AAAAA..." messages that coincide with the stuttering. No errors in the backend logs.

The same thing happens when trying to watch live vtv or play recordings from the backend. This is a remote frontend, so it's just the Pi 4 and the SD card. I've turned wifi and bluetooth off, this is hard-wired to a gigabit switch (pc works fine on same port/cable), processor doesn't get too busy during the stuttering (watching top via ssh). I tried increasing the audio buffer from 100 to 1000 but it doesn't make a difference. I don't see an options for increasing the buffer.

Ideas for next steps or is there something else I can do to better narrow what kind of issue is going on here?