You launch the store. Design looks solid, photography is clean, copy does its job. Then someone runs Google PageSpeed Insights on mobile and the score comes back at 41. It's a familiar situation, and it's almost never caused by one thing. Shopify performance is a layered problem. The theme you picked, the twelve apps installed over eighteen months, the custom JS a contractor dropped in last spring, the Liquid templates doing quiet heavy lifting on every request. Each decision stacks on top of the last. Some of those decisions help; a lot of them don't. And the frustrating part is that nothing announces itself as the culprit until you actually measure. This checklist goes through each layer in sequence, starting at the server and working outward to the browser, so you know exactly where to look before you start changing things. Start with What Shopify Controls (and What It Doesn't) Shopify handles hosting, CDN delivery through Fastly, HTTP/2, and automatic asset minification.…