I’ve had this issue on and off for the several months that I’ve owned a Synology NAS – trying to copy files greater than a few hundred megabytes in size, the file transfer will work for a few seconds, then the transfer rate will slow, stall, and eventually throw some connection error (which I forgot to write down).
Seems like this is a relatively common issue with Synology NAS and there are a lot of posts about it but not many solutions.
Just to try to add some signal to the noise, here is the fix I found.
The issue is flagged in the Tailscale Github:
https://github.com/tailscale/tailscale/issues/10408
… and the solution is actually in the Tailscale support docs:
https://tailscale.com/kb/1131/synology#enable-outbound-connections
Basically it’s just a case of running the following as root:
/var/packages/Tailscale/target/bin/tailscale configure-host; synosystemctl restart pkgctl-Tailscale.service
The docs explain how to add it to the task system, but running it manually instantly fixed the issue for me (Synology DS923+).