Ask HN: How do you sell performance?

I started a service agency that makes e-commerce sites significantly faster on desktop and mobile (Sometimes as fast as native apps). I have around 25 customers.

But when I approach new prospects, none of them care about their store's performance. I keep amazon and google's studies handy, but often the conversation ends before I can bring them up.

Highlighting revenue benefits of a fast site hasn't helped much either.

What would be a good way to educate prospects about importance of a fast e-commerce website?

If someone can point me in right direction, I'll be happy to pay for your time.