Even after the following I still wouldn't consider it usable.

# chattr +C ~/.local/share/gnome-boxes/images

# cd ~/.local/share/gnome-boxes/images

# mv win10 ../

# cat ../win10 win10

$ lsattr
---------------C---- ./win10

Here's a screenshot showing windows process manager with < 1MB transfer rates but 100% disk busy while installing MinGW w64 in windows, which is not a huge amount of data...

Any tips? Or do I just need to buy a cheap SSD and format it EXT4 just for virtual machine images?

Thanks,

Richard