WP Smush it execution time


how long does it take on average to smush images (largest files are < 2MB)?
I've been running the plugin for some 24h and it's currently at 2,500 images from 11,500 images in total.
During the first 100's pics there were savings (7,6MB) but that's seems to have stalled.

Any ideas are highly appreciated.
Many thanks,