More than 50% of ecommerce shoppers are mobile users, this is why the speed becomes an important factor in google ranking algorithm.
If you're looking for the fastest Shopify theme, I introduce PLAK, especially designed for speed.
Here is a demo store : http://plak-themev1-demo-store.myshopify.com/
Below is a speed comparison between PLAK and Shoptimized homepage speed
They are legit, it might be just an error, did you email them? They usually answer in less than 24hrs, and they also can install it for you if you have any trouble