WP Smush Pro Plugin: Slow Processing; Resmushing Images

We just started using WP Smush Pro last week, which replaced WP Smush.it since Yahoo no longer supports the free service, and we've noticed that it takes a really long time to smush even just one image. We have over 6,000 images in our media library that appear to have to be re-smushed again - why would they have to be reprocessed if we've already done that once?

Thank you.