Image compression is the advice everyone gives and rarely the thing holding a Shopify store back.
App scripts are the largest controllable cost on most Shopify storefronts, and they load on pages where their feature is never used.
Fonts block text rendering, and a theme loading four weights of two families delays the largest visible element on every page.
Uninstalling an app frequently leaves its code in the theme, so stores carry the weight of tools they stopped paying for.
Measure with field data from real visitors, not lab scores — a lab test on a fast connection hides exactly the problems your mobile traffic experiences.
Because images are rarely the binding constraint on a Shopify store. Shopify already serves images through its CDN with reasonable optimisation.
The full guide has the formats, the deadlines and the worked examples.