React compiler is awesome for minimizing unnecessary renders but doesn't help with bundle size; might even make it worse. But in my experience it really helps with runtime performance if your code was not already highly optimized.

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).

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.

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.