Windows iperf3 speed over WAN slow


I’ve setup a Wireguard site to site. When troubleshooting slow speed over VPN It turs out its only the Windows 10 and Server 2016 clients that have this superslow speeds 40-50mbit.

When running Iperf3 on Centos 8 (wireguard server) the only speedlimit is ISP over wan. Tested on public iperf3 server and over wireguard vpn to remote client.

Over LAN there’s no problem the speed is around 950mbit on both Windows and Centos 8.

I even istalled Iperf on PFsense and on LAN its great but as soon I try a public Iperf test server Windows speed is about 40-50mbit but Centos 8 is around 450-500mbit(ISP limit).

I’ve googled but to no real answer except TcpWindowSize but it’s very unclear how to edit this.

The setup is Windows server 2016 Hyper-V server running PFsense, wireguard, Vault Server, Win 10.

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iperf3 -w 64KB

When I run Iperf with the -P option and choose to run 10 parallel the sum was around the ISP speed limit. And when I tried the SMB file share over Wireguard VPN the speed was great around 25-30MB/s istead of 5MB/s. I don’t know why it started working as I expected thou.