Fossilize influenced by I/O load on unrelated drives

Your system information

• Steam client version (build number or date): latest (Jan 16 2022)
• Distribution (e.g. Ubuntu): Gentoo
• Opted into Steam client beta?: [Yes/No] No
• Have you checked for system updates?: [Yes/No] Yes

Hardware:
3900x
64GB RAM
Rest described below.

When using the following setup with an I/O load fossilize will go down from maxing my processor to using only one core at 50% to having no CPU usage at all.

Setup:
BTRFS RAID0 (2x512GB /)
BTRFS RAID0 (2x10TB /mnt/RAID)
BTRFS Single drive (1x3TB /mnt/RAID/3TB)
Have steam install and download games in /home/user

How to replicate:
Start processing vulkan shaders;
Start transferring files from /mnt/RAID/3TB to /mnt/RAID (this shouldn't cause any problems as the I/O is not on the main drive).

Fossilize now vanishes from system and stops working until the transfer is completed, cancelled or paused.

Reason for bug:

The I/O is not on the main drive that steam is installed on, so it shouldn't affect fossilize. On my 3900x the load average is 3. Plenty of CPU for fossilize to run.

Pausing the transfer results in the CPU suddenly hitting 100% with fossilize using all cores after a second or two.

Reproduce Vulkan shader processing while transferring files between the two unrelated RAID mounts described in the report. Start by tracing Fossilize's processing and I/O behavior during that transfer; done means processing continues at full use instead of stopping until the transfer is paused or completed.