Speed difference site2site and vpn

I have a site2site network which consists of a 1 server and 1 client (both MT300N-v2). The site2site speed is okay being around the max upload speed (20 Mbits).
When I configure the MT300N’s as wireguard-server and client the speed drops with around 25%.
See enclosed figure (first test is s2s, 2nd test is wg server client.
Any ideas why that is?

You need to make sure that your network environment is completely consistent, and this is best done on a LAN.

I’ve tested it 10 times in a real life situation (over the same networks) and the client server connection is consistently about 20% slower than the s2s.

This seems an interesting finding.
I have no idea.