Source too slow means a lack of bandwidth in the network or the NAS/Server device hosting files being unable to stream the file fast enough. I see this occasionally with very large ISO rips that I've made (70-80GB size) but it clears after 5-10 seconds. It can also show up if the server is trying to transcode content on the fly and not quite keeping up .. which I occasionally see when using Plex as the back-end server (with the Plex client add-on in Kodi).
The "hdmi_enable_4kp60" option is rarely needed and causes much higher current draw on the PSU and will need an HDMI cable certified for 4K60. Cheap(er) cables will often give blank screens. Under-spec PSUs can do the same. Practically all media is [email protected] or 4K30, only GoPro files and content people have deliberately encoded in 4K60 requires it. Most users do not need it (which is why it's forced as a default).
I don't use DLNA and have never used Jellyfin, but I occasionally use Plex and it works fine. If you are using WiFi .. that's probably the cause of all playback issues. Connect a cable and test again. WiFi is rubbish for streaming media and that's a near-universal rule and nothing specific to RPi4, although RPi4 having no external antenna and being often installed in a nice shiny metal case doesn't help.