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.
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