Why vpn connection virtual interface just has 10Mbps?

I just recently noticed that the network interface created by tailscale and openvpn has speed of just 10Mbps. Does anyone know why ? and does it slow down speed? and if it can be increased ?

This has nothing to do with Nextcloud. Nextcloud does only show you what the underlying Linux system is reporting. I’m not an expert on this, but on virtual interfaces, the actual speed you can get is afaik not limited by what the interface is reporting. But it’s of course limited by many other factors, like the CPU and of course the speed of the physical network adapters, if the traffic is leaving your system. Also the encryption type and the ciphers you are using with OpenVPN can affect the speed… etc, etc…