Judge a theme by how the real stores running it perform in Google’s field data, weigh the sample size behind each score, then protect that speed as you add apps and images. Ask ten store owners what makes a Shopify site fast and most will point at the theme. The theme you choose sets the speed ceiling for everything you build on top of it, which makes it one of the few early decisions that is genuinely hard to reverse later. The trouble is that almost every theme is marketed as fast, and the demo store you click through in the Theme Store is the least reliable place to judge it. This guide walks through how to tell a genuinely fast theme from a well marketed one, and which themes hold up when you look at real shopper data.