Guest stuttering under high disk IO (virtio)

Performing io intensive tasks on virtualized Windows causes the system to visually stutter. I can often reproduce the problem by running fio on windows:

fio --randrepeat=1 --ioengine=windowsaio --direct=1 --gtod_reduce=1 --name=test --filename=\\.\PhysicalDrive0 --bs=4k --iodepth=128 --size=4G --readwrite=randread

While the fio command is running, moving the mouse pointer will be be laggy. The stuttering does not appear with iodepth <= 32 . The stuttering also manifests while playing games, the music and video pauses for a fraction of second in a playable but disturbing way.

Here are my system specs:

Host OS: archlinux
Guest OS: Windows 10 Enterprise
qemu version: qemu-git 8:v4.1.0.r1378.g98b2e3c9ab-1 (from AUR, compiled with -march=native)
CPU: AMD Ryzen Threadripper 1900X 8-Core Processor
Huge Pages: vm.nr_hugepages=4128
Disk: nvme type=raw, io=threads bus=virtio
GPU (passthrough): Radeon RX 570

Here are some fio test results on my windows guest:

[size=512M,iodepth=1 -> min=30k,avg=31k,stddev=508]
[size=2G,iodepth=8 -> min=203k,avg=207k,stddev=2.3k]
[size=2G,iodepth=16 -> min=320k,avg=330k,stddev=4.3k]
[size=4G,iodepth=32 -> min=300k,avg=310k,stddev=4.8k]
[size=4G,iodepth=64 -> min=278k,avg=366k,stddev=68.6k] -> STUTTER
[size=4G,iodepth=64 -> min=358k,avg=428k,stddev=52.6k] -> STUTTER
[size=4G,iodepth=128 -> min=92k,avg=217k,stddev=185k] -> STUTTER
[size=4G,iodepth=128 -> min=241k,avg=257k,stddev=14k] -> same config as above, but no stuttering

The min and avg values are the bandwidth values reported in KB/s by fio. You can see that, when the stuttering occurs, the stardard deviation is high and the minimum bandwidth is way below the average.