would lazy-loading img src negatively impact SEO
I'm working on a shopping site. We display 40 images in our results. We're looking to reduce the onload time of our page, and since images block the onload event, I'm considering lazy loading them by initially setting img.src="" and then setting them after onload. Note that this is not ajax loading of html fragments. the image html along with the alt text is present. it's just the image src is deferred.
Does anyone have any idea as to whether this may harm SEO or lead to a google penalty box now that they are measuring sitespeed?
Answer
Images don't block anything, they are already lazy loaded. The onload event notifies you that all of the content has been downloaded, including images, but that is long after the document is ready.
It might hurt your rank because of the lost keywords and empty src attributes. You'll probably lose more than you gain - you're better off optimizing your page in other ways, including your images. Gzip + fewer requests + proper expires + a fast static server should go a long way. There is also a free CDN that might interest you.
I'm sure google doesn't mean for the whole web to remove their images from source code to gain a few points. And keep in mind that they consider anything under 3s to be good loading times, there's plenty of room to wiggle before resorting to voodoo techniques.