I’ve started a new WordPress Site with fresh Catalyst theme install. Ran a performance test via gtmetrix.com and got an F-score for scaled images. They are resized instead of scaling.
We’ll need to fix that inside the site itself. We’ll likely re-build it inside Gutenberg.
For your site, you’ll want to replace those demo images with your own images. When you do that, make sure you upload images close to the size you want them to display as. That will prevent them from scaling so much.