Windows SMB on gigabit LAN goes 1000 times slower when I turn on VPN

I noticed that my Windows SMB performance on my gigabit LAN normally runs at about 950 Mbps for big files. But when OpenVPN is on, it drops. Not immediately, but soon. In like 5 seconds, close to the end of copying a 1 GiB file, it drops to lower than 1 Mbps. When I turn off OpenVPN it immediately jumps back to 950 Mbps. How to fix it? I’m on Windows 11.