yeah, at ~100mbit speed tests you almost certainly have a bottleneck somewhere. 100mbit ethernet adapter is a good place to check. Also verify the ethernet cable you’re using. I’ve found some “cat 5e UTP” were missing the “twisted pair” part and that prevented it from reaching gigabit link speeds. The next is any switch you may have in line between phone and router. Next is the router. If the router has 100mbit switch ports, you’re bottlenecked no matter what you plug into it.

NAS vs Jellyfin isn’t really how to look at it. You still need to have the files stored somewhere, and Jellyfin can just access the files from wherever you store them. As others say, Jellyfin adds some convenience. Think of it like Netflix but for your local files. You can install an app on your phone, laptop, tablet, or just access Jellyfin’s built in web interface on laptop/etc… It pulls down thumbnails and show information automatically, and you can set up different accounts/profiles to track show progress and favorites for multiple people.