tls-auth ta.key makes very slow openvpn

Daimroc

I am doing tests about speed when I want to transfer a file. I have a virtual machine with Windows server 2012 as server and in my host Windows 10. I have notice that when I use tls-auth ta.key as extra security, the transfer of a big file is about 10MB/s. However, if I don't use this, I get a transfer of 40MB/s.

So I would like to know if I am doing something wrong or really tls auth can make slow down the speed about 4x.

Thanks.