Glossary
Headless commerce
The storefront and the commerce backend are separate systems talking over an API.
What it is
A headless setup separates the shop customers load from the system holding products, orders and payments. The frontend is its own application; the backend keeps doing what it did, exposed through an API.
Why it costs you money
It removes the theme layer that caps performance on most stores and gives you control over what the browser downloads. It also adds a codebase to maintain, which is why it is the right answer for some stores and an expensive one for others.
How to check it yourself
Check whether your constraint is a backend limit or a frontend one. Poor TTFB means backend; good TTFB with poor LCP and INP means the browser is doing too much, which is what headless addresses.
Where this matters