[bastawhiz]

When you compile JSX to JavaScript, it produces a series of function calls representing the structure of the JSX. In a recent major version, React added a new set of functions which are more efficient at both runtime and during transport, and don't require an explicit import (which helps cut down on unnecessary dependencies).

[silverwind]

You mean the automatic runtime introduced in 2020. It does not have any impact on the performance, it's just a pure developer UX improvement.

[bastawhiz]

It improves the bundle size for most apps because the imported functions can be minified better. Depending on your bundler, it can avoid function calls at runtime.