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.
Do you get better speeds transferring over Tailscale when both machines are on the same LAN?
I have no problems pushing 400mbit over Tailscale but it does use a ton of CPU so maybe the Synology CPU is the bottleneck if you have similar speeds after eliminating your internet connection.
This may be a silly question, unless is isn't. Are you sure that your maximum upload speed is 300Mbps? Your maximum upload speed can be different to your maximum download speed. https://speedof.me can help you check.
If that's not the answer, sorry for possibly being overly simple, but some people might not realise.
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
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?
I have no experience with NAS but does it have an HDD? Maybe the read/write speed is the bottleneck. Again take this with a grain of salt since I do not have any experience with running a NAS.