Hm, that sounds vastly jankier than Plex per your description. I have never had to wait for any UI element to load in plex, local or remote, including thumbnails. Everything is almost always instantaneous. I've tried the demo instance of Jellyfin a few times to see if it has improved and my impression was that it's a complete joke, not a serious plex alternative.
I have the opposite experience. I tried to setup Plex a few times but always ran into issues with getting ads, having to make a Plex account, and dealing with free/paid features. I just couldn't get it to work without ads and all the hassle. Instead I installed Jellyfin on a NUC running Docker, downloaded the app from the play store onto my TV, and it just works, no delays, no lag.
I have Jellyfin on my local network and it works like a dream. I used to use Plex and, at least for my needs, one was a straightforward replacement for the other. The web UI is clean and snappy and just works.
