VPN connections like OpenVPN and WireGuard use virtual network adapters in order to establish a connection. It is used to establish a bridge between two different networks, which allows communication between them. By default, OpenVPN uses TAP-Windows Adapter V9.
The TAP adapter can cause issues for some people due to the way OpenVPN exchanges packets with the TAP adapter, which can cause bottlenecks to occur which in turn can lead to speed limitations even though your processor in theory should be able to encrypt/decrypt internet traffic faster than it currently is.
Wintun is a new virtual adapter originally designed for use with WireGuard, and is a lot more lightweight and in some cases more efficient than the TAP adapter previously in use for OpenVPN. Starting with OpenVPN 2.5, OpenVPN can establish a connection using both the Windows TAP adapter and Wintun adapter, allowing users to select the one that works best for them.
We recommend using the Wintun adapter when possible.
This setting is only available on Windows.