Windows 10 Guest Performance Degrades to unusable

[TSD]

I use an Ubuntu 18.04 host on a Supermicro system with 64G of RAM and a pair of Intel(R) Xeon(R) CPU E5-2695 v3 @ 2.30GHz.

I previously had a Windows 10 VM which I was happy with. I recently upgraded my Windows Guest to Windows 10 build 1809. At the same time I upgraded Virtual Box to 6.0.12. Performance is good when I start the VM but gradually over a period of several days the performance with keyboard and mouse inputs degrades to very laggy and difficult to use. If I reset/restart the VM without shutting it down, performance issues continue. But if I shut down the VM completely, and start again it is ok again.

Any suggestions? Log is attached.

[foxquakes]

Noticing the same thing myself with the 1903 update (perhaps that's what you meant). I hope this is a widespread issue that can be tracked down and solved quickly.
My host is also 18.04. I updated my vbox installation and addons to rule that out.
It doesn't take days, more like 0.5-1 hour of idleness drags it down to hell and it never comes back. For some reason staying connected to the guest via RDP, it remains stable, but the moment I disconnect it's doomed.

[socratis]

@TSD

00:00:01.665863 [/Devices/hpet/0/Config/] (level 4)
00:00:01.665865   ICH9 <integer> = 0x0000000000000001 (1)

Any particular reason why you decided to override the template defaults, and go with an unsupported, experimental, OSX-guests-only option? Please change it back to the default PIIX3. Do not change the defaults unless you know what you're getting into.

00:00:01.666175 [/Devices/virtio-net/0/LUN#0/] (level 4)
00:00:01.666177   Driver <string>  = "IntNet" (cb=7)

Again, why did you go with the virtio-net? Because you thought it would be faster?

That's all I could find worth mentioning...