You can disable transcoding, but then your users may not be able play some videos on some of their particular devices. MKV is just a container. The codec of the video, audio, and subtitles are what matter.
Jellyfin 10.10.7 (Docker)
Ubuntu 24.04.2 LTS w/HWE
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
4x WD Red Pro 6TB CMR in RAIDZ1