In my experience that kissmetrics blog is a load of nonsense in practice.

We massively reduced load time on the all the key pages from 4 secs to roughly <400 ms avg. No 25% bump in sales for us. At the time the client sold about 50-80 things a day on thousands of daily visits. A month later, they sold about 50-80 things a day. This was a fairly standard website similar to booking.com or any search/book site, basically location aware searches with a landing page for each venue and a "book" CTA.

I did a similar thing with their mobile site a year or two later, massively reducing the page-weight and reducing loading times on the devices I tested from seconds to again sub-second load times. Again no discernible bump.