Headless is not faster by default and not better by default.
Headless removes the theme layer and gives you full control of the front end, at the cost of building and maintaining everything the theme provided.
It is not automatically faster — a badly built headless storefront is comfortably slower than a well-built Liquid theme.
The strongest genuine cases are unusual front-end requirements, a shared front end across several backends, and existing in-house React capability.
The hidden cost is everything the theme ecosystem gave you free: app blocks, the theme editor, and merchant self-service.
Not inherently. Headless gives you control over what loads and when, which makes excellent performance achievable.
The full guide has the formats, the deadlines and the worked examples.