[for_xyz]

> So... it's both NTFS and the Windows "subsystems."

Ok, probably both on default windows installation due legacy and backward compatibility with dos names [1].

https://docs.microsoft.com/en-us/windows-server/administrati...

I remember this issue when creating few million of files inside one folder and it was extremelly slow because of 8dot3 name creation. It has to go through each filename to generate short name O(n) when this legacy feature is enabled.

After disabling 8dot3 there were no performance issues anymore.

> I have no experience with BTRFS, but that doesn't prove anything without knowing more details (the overhead introduced to make it work)

I tried to point out following: Ext4, Btrfs, Zfs, any other UNIX filesystem will be slow under Windows. NTFS or any Windows first filesystem will be slow on Linux. There is just too much of the differences in OS architecture between the NT and Linux.