Server Version#: 1.25.3.5409
Player Version#: 6.9 build 14
Kind of a PSA since I think my issue is resolved. Posting in case it helps others.
Plex Media Server version 1.25.2.5319 works fine, version 1.25.3.5409 causes significant buffering issues with Roku Ultra
Synology NAS → Plex Server (Win 11, Dell Optiplex 7050 Micro Desktop, Intel i7, 16GB RAM) → Roku Ultra
All wired connections, no WiFi.
I was on the new version for ~24 hours but experienced significant buffering issues playing videos from the Synology NAS on my home LAN. I rolled the server back to version 1.25.2.5319 and, although I really don’t think anything else was at fault, also rebooted my router, switches, the Win11 Plex server, etc., etc. So far things seem fine again.