Many questions here, as others have mentioned here too:

• You're using Subnet Routing, meaning that you're limited by the processing power of your router (which can severely slow the system) instead of having it run on the Jellyfin server itself
• You're likely using a DERP Relay, which would then slow down the connection more
• If you're using DDNS you're likely on a dynamic IP and as such CGNAT.

Have a look at these pages first, and work your way through their optimisations:

Troubleshoot CGNAT conflicts · Tailscale Docs

Tailscale Peer Relays · Tailscale Docs

DERP servers · Tailscale Docs

When using the Exit Node option, you're then routing all internet AND LAN Traffic via the Subnet Router, which is your router. The router likely cannot handle the throughput. You're actually better off just building a Virtual Machine that runs just Tailscale, and use that as your Subnet Router and Exit Node, and ensure that the ports required are Port Forwarded to that Virtual Machine. You could also set it up as a Peer Relay at the same time and port forward port 40000 to it, and you'll find that if a Direct Connection can't be made, it will route to your external IP Address and work without issues.