rawhide ext4/virtio FS performance hit (10x slower) ?

[Jim Meyering]

Today I noticed that some tests from coreutils' test suite
were taking far longer than they used to on rawhide.
For example, run this command in an empty directory:

  seq 200000|env time xargs touch

it takes < 5s on F17 3.3.4-1.fc17.x86_64 ext4/sdb8/SSD, yet
it takes 47s on rawhide 3.4.0-0.rc5.git5.1.fc18.x86_64 ext4/vda1/SSD

10x performance hit... ouch.

Does anyone know why?