Be careful with the smush plugin, my site was running slow and after checking I found that the culprit was that my wp_postmeta database had thousands of lines with wp-smpro-smush-data lines in.
Disabling the plugin and deleting it still did not remove the lines from the table.
You have to remove them manually (I used phpMyAdmin) and once I'd done it my site went from 7 or so seconds to 2 on a page load.
Good plugin for compressing images (albeit it takes ages!) but once it's finished make sure you remove it and delete those lines from your wp_postmeta table or it will have the opposite effect of what was intended and will slow down your WordPress site.