15 MiB/s is trivially handled by any CPU you're likely to run. Indeed 100 MiB/s seems reasonable. 15 MiB/s cap seems either the protocol being used is doing too many round trips (assuming the machines you're testing with are far apart) or the network that's being set up requires routing through Tailscale's infra for hole punching.
It sounds like the traffic gets routed through a Tailscale relay because all attempts at direct connection failed. A direct connection would have been as fast as a direct connection.
Ok a 12% differential on a LAN is kind of surprising. I wonder what Tailscale could possibly doing that would be causing this issue because aside from the control plane I don't believe they're in the data path all that much. Maybe WireGuard on Windows isn't as optimized as it is on Linux?
IME it adds about (at least) 1ms of latency over local networks. You should be able to use a different dns suffix to use the LAN interface instead of Tailscale.
