It didn't make the site measurably slower. We specifically tested this. We already optimize in other ways and lazy-load most of the property images. The only ones that load right away are the first 3 property images in the list.
This is impossible to achieve in the real world. Load times have a lot of variance. Our tests were performance neutral after the change.
In a vacuum yes more data = longer load, but that's not how it plays out in a real browser. Things load in parallel, things block in certain parts but not others, etc. So yes, you really can sometimes send more bytes with no impact on total load time. It just depends on what they are, where they are, and how they load.
