I wanted to ask if anybody has been in a similar situation as me. Let me briefly explain my topology and the problem that has arisen. I have an on-premise site with mainly Windows resources and an up-to-date Forti firewall as the central communication device. This site is connected through a partner interconnect created according to Google's design, using 2x10gb lines. From Google's side, I use cloud routers, shared VPC, and resources in projects. All the Windows OS versions I've tested are Server 2019, 2022, 2016, and Windows 11.

In Linux to Linux communication, there are no problems - good bandwidth and no packet drops. However, in Windows communication, I only get up to 50 Mbps when measured with iperf, and the overall communication is bad. I believe this is connected to the TCP window size. When I try to increase the window size in iperf, the first line shows good bandwidth, but it immediately drops again to around 47-50 Mbps.

I've tried different combinations of ECN, scaling, and auto-tuning with no success. In fact, I only made things worse. Any ideas would be appreciated.