In the Jellyfin app on your phone, click on Settings > Playback (not the dashboard) and set the max transcode resolution to 1080P. Right now the output of the transcode is 4K.
Jellyfin 10.10.3 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)