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.
Have you tried using ZeroTier? I don‘t have much experience using Tailscale but have been using ZeroTier for two years now without any problems. Runs great on Synology