From my understanding the dh key is used to transfer keys when the connection is made. Which encryption affects OpenVPN speeds? AES256, RSA4096? Ect.
OpenVPN establishes a TLS connection to transport packets, so the answer to this would be the same as TLS.
RSA and DH is only done once at the start of connection, so while they greatly affect initial connection creation, they don't take part in further encryption during the session. So the only encryption that's relevant in long running connection is AES, which is really fast in modern processors that supports hardware accelerated AES, and is usually fast enough that it's not going to be the bottleneck even in processors that doesn't support AES acceleration or when AES acceleration is not used.
Probably the main determinants of OpenVPN speed is:
- the network bandwidth and latency between the VPN terminators
- whether you use TCP or UDP as the transport. For the purpose of OpenVPN, UDP usually performs better because missing packets would not delay the delivery of other unrelated packets
- the processing capacity of the VPN terminator hardware is usually not the main bottleneck, unless you're connecting multiple medium or large networks