Smush Pro very slow, nothing on CPU

Running WP Smush Pro v2.3.1, on a local host.

It smushes about 1 image per 3 minutes. CPU remains un-busy. No apache2, mysqld or other executable taking any appreciable CPU.

What's it doing and why's it so slow?

Super-smush enabled, but same problem when disabled.
Automatically smush my images on upload enabled.
All other options disabled.
Only plugins enabled are Dashboard, Smush, and Hummingbird.
Default theme.
WP 4.5.2