I'm only getting an average of 9.21 Mbps from my Synology NAS to my remote Mac device. My NAS is connected to LAN with an internet speed of +-300Mbps. I don't expect 300Mbps through Tailscale, but a 80-100Mbps would suffice.
The tailscale guys are very responsive if you send them a question.. It's meant to be P2P but when it can't i believe it uses some kind of relay proxy server so at a guess i'd say it's that. Personally i run a subnet router on my LAN and not on individual devices.. maybe you could try that instead?