OpenVPN over asynchronous WAN lines: expected speed? Overhead?

[Cabledude]

Hi all,
I am setting up an OpenVPN Remote Access Server on pfSense. Both endpoints have an asynchronous WAN line. What speed can I expect?

TL;DR
When going from Site A (40mbps upload) to Site B (50mbps download) should I get 40mbps transfer speed? I only get 6mbps and the SG-1100 is doing 12% CPU.
end TL;DR

Site A: Server side
WAN line 500 down / 40 up

Site B: Client side
WAN line 50 down / 20 up with speedtest, the ISP plan is 40/5 but apparently we are getting more than we bargained for.

Expected transfer speeds for downloading from site A to Site B? There are different opinions re overhead and asynchronous lines, ISP's that throttle VPNs etc.

In a first real test I got about 6 mbps download from Site A to Site B.

Connection details:
Site A
Ziggo 500/40 WAN line
Netgate SG-1100, should do 60-70 mbps VPN
iperf3 test over LAN lab setup for the SG-1100: 60-ish mbps speeds
When connecting over the internet 6mbps, CPU was at 12%
OpenVPN server using local user auth + user cert
21.05.2 fully updated

Site B
Ziggo 50/20 WAN line
ISP modem/router (Connectbox)
Macbook Pro Late 2013 using a 702 mbit Wifi connection to LAN network, easily handling the full 50mbps.

Before starting all this I was expecting A->B downloads to be 40mbps (Site A upload speed is 40mbps and Site B download is 50mbps). But I am now wondering if this is realistic.

Thanks for any insights!

Pete