I found a more convincing solution to my "almost frozen" Win 10 qemu/KVM issue.
My CPU is an Intel "Kaby Lake" generation, specifically:
$ inxi -C
CPU: Topology: Quad Core model: Intel Core i7-7740X bits: 64 type: MT MCP L2 cache: 8192 KiB
Speed: 4501 MHz min/max: 800/4500 MHz Core speeds (MHz): 1: 4501 2: 4500 3: 4502 4: 4500 5: 4500 6: 4500 7: 4500
8: 4500
I have the CPU set in VMM to "Copy host CPU configuration".
But some googling turned up
this post with the solution to edit the XML to add
<synic state='on'/>
<stimer state='on'/>
in the "hyperv" paragraph of the XML for the VM. I did that three days ago, gave it only 2 CPUs, and 4 GB of memory, and it has been running problem-free for three days. Apparently something in the qemu 5 update exposed the problem and this change to the XML fixed it.
Now it actually is solved.