This is Song Fei from Giraffly company which develops apps for Shopify.
Shopify recommends using 2048 x 2048 pixels for square product photos, which show up well on mobile and desktop alike. These images will be high resolution, giving your store a professional look and helping you sell more, and the pixel count is high enough that you’ll have exceptional zooming capabilities (which kick in once the images are at least 800 x 800 pixels).
And if you want to boost your page loading speed. Our app can help you perform that. We developed an app called Page Speed Boost
It uses the cheating latency which helps you preloading a page right before a user clicks on it. There is a time between a user hover the mouse over a link and actually click on it. That time is so quick and people won't even notice while our App detects and takes good advantage of that time and do a just-in-time preloading for that page. So when you click on that link, that pre-loaded page opens instantly. This principle works on mobile as well, as when a user touching a link and before releasing it leaves PageSpeed Boost a good time to preload that page.
And you can see more instruction here: Page Speed Boost. It's an amazing app!