How can I troubleshoot the slowness?

[RelatableQuery]

JellyFin is running in a docker container on my Synology Nas. I consume it though either the web interface, an andorid app or the JellyFin for Kodi addon.
In all three instances browsing is very sluggish. The Web UI takes 10-20 seconds to load, in Kodi it could take 30 seconds to show the content, etc.
Once the media starts playing there are zero issues.

Am I just impatient and spoiled by using a local library on Kodi, or are there some tweaks I can make to speed things up?

[bitmap]

Local or remote when accessing? Is there a difference in speed between the two when you access in-network vs. external? Generally a faster internal drive where Jellyfin is installed (i.e., not a HDD) will lead to faster loading and navigation. Ideally media is on HDDs while your Jellyfin install is on an SSD or NVME.