Background optimization is great, because it lets you upload an image and get to work without waiting for EWWW IO to finish compressing an image.
But it will also send an extra HTTP request for each image upload, and then that "request" will be responsible for optimizing the image plus all the resizes that were generated. Parallel optimization takes that a step further, and breaks each individual image resize into separate requests, and then runs those 5 at a time. Sounds great, right!
Usually, but it can also be a big mess! We've run into issues with security plugins thinking EWWW IO was doing nefarious things, and all sorts of fun.
If you need to disable background/async mode for any reason, simply define EWWW_DISABLE_ASYNC in your wp-config.php file:
define( 'EWWW_DISABLE_ASYNC', true );
That's it, hope you like slow uploads! Ok, hopefully they aren't too slow, but when you do that you'll get to see first hand exactly why background mode exists. But sometimes, you have no other choice, so EWWW IO gives you one instead.