The backend is written in C# and .NET. The CIL runtime on Linux is shockingly slow in comparison to Windows. Ontop of that, it feels as though their DB backend is kinda meh.
The client itself is okay, but there's a reason people wrote alternative clients for it already. Unfortunately, the only way to make it actually really smooth is to throw a crapton of resources at it.
That said, its free, open source, absurdly featureful. So I don't want to complain too much - after all, I can't do it better myself x). There's probably other factors I am not taking into account about the laggyness. Over here, its sometimes a little slow, especially when it works with my LiveTV setup (SAT>IP server into TVHeadend into Jellyfin Plugin into Jellyfin) but for most other things it's speed is... okay. o.o But not "fast" either.