Image Optimization can be time consuming, maybe you've noticed that by now? Even beyond the optimization process, trying to scan entire folders looking for images is not something the default PHP configuration can handle.
You might already know how to fix that, but EWWW IO likes to do some things for you, if it can. There is a nifty little function called set_time_limit() and it allows EWWW IO to eliminate the time limit within PHP. BUT, it only works if your webhost allows the set_time_limit() function, and some don't like that.
Even worse, some webhosts use non-standard ways of restricting the set_time_limit() function, and they don't care to share how in the world they do that. If you activate EWWW IO on one of those webhosts, and turn on Scheduled Optimization, you'll get constant errors in your logs, and possibly elsewhere.
To prevent EWWW IO from ever using the set_time_limit() function, simply define EWWW_IMAGE_OPTIMIZER_DISABLE_STL in your wp-config.php file:
define( 'EWWW_IMAGE_OPTIMIZER_DISABLE_STL', true );