Many users reported high memory/RAM usage, some 8GB+.

In my case gone from 1.5GB+ to 400MB or less on Raspberry Pi 4.

Adding MALLOC_TRIM_THRESHOLD_=100000can make a big difference.

With Docker:
Add to your docker-compose.yml and docker compose down && docker compose up -d

...
environment:
  - MALLOC_TRIM_THRESHOLD_=100000
...

With systemd:
Edit /etc/default/jellyfin change the value of MALLOC_TRIM_THRESHOLD_ and restart the service

# Disable glibc dynamic heap adjustment
MALLOC_TRIM_THRESHOLD_=100000

Source: https://github.com/jellyfin/jellyfin/issues/6306#issuecomment-1774093928

Official docker,Debian,Fedora packages already contain MALLOC_TRIM_THRESHOLD_.
Not present on some docker images like linuxserver/jellyfin

Check is container (already) have the variable
docker exec -it jellyfin printenv | grep MALLOC_TRIM_THRESHO LD_