Unfortunately, when left activated WP Smush.it slows down your site…so it has the opposite effect it is meant to achieve!
And WP-Smush.it, a Yahoo service behind the scenes that does the image processing, seems to go in and out of ‘working’ properly quite a bit over time.
What We Recommend
So we recommend you only activate this plugin as needed (when adding images) to get the benefit of image compression for new images that are uploaded, then deactivate it immediately when done uploading images since it will slow down your site.
Use Only As Needed
Since activating and deactivating the WP Smush.it plugin every time you want to add a post or page is a hassle, you can leave it deactivated most of the time, then run a ‘bulk Smush.it’ once in a while to compress any images that you may have missed.
Bulk Smush.it will run through all of your images at one time and compress (or even re-compress) any images it can. Bulk smushing can take a few minutes to process.
Warning: I’ve found that Bulk Smush.it often doesn’t work either (it usually times out), so proceed with caution, then ‘disable’ when not in use.