I am running out of idea so any suggestion would be welcome.
The webmail is extremely slow, particularly on first load, or when logging in, then suddenly becomes fast again and a few minutes later very slow. By very slow, I mean that when connecting to the webmail it can take more than 1 minute before anything displays.
Smartermail enterprise 15.1 (updated yesterday) running in a VM on a very fast machine, very fast SSD, Windows 10 Pro, 100GB free on the C drive, the CPU usage is close to zero, the network also. The IIS log file doesn't show anything abnormal, I seem to be the only IP connected, and the smatermail admin says the same. Doesn't look like a DDOS. I have been running smartermail for years without a problem, and moved to Win 10 6 months ago (the machine is likewise 6 months old, fresh windows install), however the problems started only a few days ago.
I tried uninstalling smartermail, rebooting and reinstalling it, no change in behavior. No antivirus running on the machine, outside of windows defender, but it doesn't seem to do anything. Other software installed on this machine:
OpenVPN (not running), Chrome, FileZilla Client, RdpGuard, Spybot Anti-Beacon, VLC, Winrar. And that's it.
I can't think of anything else to try
[Edit] Also the problem is the same whether I connect to the webmail remotely or from within the VM (via RDP). And the VM itself is very responsive. And other websites running in other VMs on the same machine do not have any problem. It seems to be smartermail specific.
[Edit2] Adding one more piece to the puzzle: I created another website in the same VM which doesn't block like the webmail, so it doesn't seem to be IIS either.