I've been trying to upgrade from 10.10 to 10.11 for a while now, as the Android TV app keeps nagging me, and every attempt ended with impossibly long library scan times.
After some thorough investigation, it appeared that a Home Videos type collection causes unending scan (yet to be solved), but also that Jellyfin does a lot of writes to the config directory (either database or metadata or both). Mine's on spinning media part of a ZFS pool. I tried a few performance tuning options, such as testing the config dir with recordsize (similar to block size) of 4K, 8K, 64K, 128K and library scans fell from 30-40 minutes down to 8-13min with 4K-64K. The ZFS tuning wiki suggest 64K recordsize with LZ4 compression for SQLite workloads such as Jellyfin. That seems to work as well as 4K and 8K but likely is faster when reading thumbnails and such.
Note that upgrading to 12-rc3, which is supposed to speed up library scans did not improve scan times for me. Optimizing config/database write speed did. I cross-checked the culprit by experimenting with moving the config dir to NVMe and RAM. Both of those got the scan times down to 8-9 minutes compared to the optimized spinning media's 12-13.
So if you had upgraded (or about to) to 10.11 your library scans are (about to get) dog slow and your Jellyfin's config dir resides on spinning media, optimize its write performance for SQLite.
I've always kept my apps and databases on solid state media, putting them on spinning rust, even in a well tuned ZFS pool with lots of disks is way too slow for their (admittedly inefficient) database.
This is true but it's nice to have the whole app, data and database in one place, going together, snapahotted together, backupable together. It's slower for sure. With a large pool (more disks) and especially with the magic of SSD cache, database reads fly and reads are the majority of the loads in my heads. In the future I would put root on ZFS as well and either do SSD cache or have root on ZFS SSD pool that gets send/recved regularly onto the spinning pool so it's easy to restore when needed.