Bertel

Thanks for the advice, but as stated in prior posts, several windows drivers, even different NICs were tested without any change. Furthermore, as mentioned several times, the 3 Gbps were reached with a completely new install of current Windows 10 2004, whereas a clean Windows 10 1909 reached around 6 Gbps, and Ubuntu-to-Ubuntu reached ~9.4 Gbps, all on the same hardware, all without any AV software. Zonealarm definitely made bad readings worse, but the main culprit appears to be Windows.

True. However, as much as going from 2 Gbits to 3 may feel like an improvement, let's not forget that Windows should do much better, and as my tests show, a previous version did. Also, Kaspersky did not affect the speed.

I am hunting down a similar perplexing situation. I have a powerful (16core Threadripper, 128G of RAM) Windows 10 machine connected to a hi-power (32core Threadripper, 256G of RAM) Ubuntu Linux machine via 10GBE network. A while ago, network speed between both machines became very slow. I booted the Windows box with Ubuntu, and iperf3 speed between the two machines was at 9.41 Gbit/sec. That ruled out hardware problems. I did a completely new install to Windows 10 Pro, Version 2004, OS build 19041.610 . That brought the iperf3 speed to around 2 Gbit/sec. Booting in Safe mode and shutting down all services that could be shut down did not improve matters until Zonealarm was shut down. Speed now at around 2.4 Gbit. After I completely removed Zonealarm and replaced it with Kaspersky, iperf3 speed was at around 3 Gbit/s, but that was as fast as I could tweak it. HOWEVER, when I booted the machine from an emergency a Windows 10 Pro, Ver 1909 build 18363.1139 install, the speed jumped to 6.14 Gigabit/second. All possible tweaks (jumbo frames, large buffers, etc.) applied with near zero impact. Changed and updated drivers, even swapped 10 GBE NICs on the Windows machine (Asus/Aquantia XG-C100C and Intel X540-T2, both in a x8 slot of the Windows machine). The fact that the same hardware runs at close to wire speed with Ubuntu on both sides, and that Ubuntu to Windows 10 Pro, Ver 1909 runs twice as fast as Ubuntu to Windows 10 Pro, Version 2004, tells me that something is wrong with the latest Windows. Reported to Microsoft, did not hear back. P.S.: The Zonealarm impact is perplexing. The 10GBE NICs and network segments were excluded from Zonealarm. Also note the speed difference between TURNING OFF Zonealarm and REMOVING Zonealarm.