In reply to Ciprian Popescu.

If anyone is loading thousands of images on a webpage, I don't think their primary speed-related concern is going to be with a PHP check for the loading attribute.

Not performing the check would conflict with hard-coded loading attributes on existing images. It would be poor coding practice to attempt to push an attribute to an HTML element without first making sure it doesn't already have that attribute.

As for the priority between core and plugins, it will ultimately depend on the priority used for filter callbacks.