As I see, request headers are retrieved from the server params (ServerBag), so they can be lazy initialized on the first use. This can be a perfomance improvement for all requests which does not operate with request headers (almost all?). Moreover, some use-cases of headers can be simply replaced with server params use-cases.

So I think there is a great opportunity to improve component perfomance, but "headers" are public property of Request class, so in order to use this improvement we must set it to protected and provide a getter with lazy initialization(or provide magic __get() with "headers" property handling - bad way, IMHO). This is BC breaking improvement.

What do you think about that?
If you agree with me, I can provide a PR for this improvement.

Also, I can admit that headers are poorly architectured today(ex. handling cache-control header in HeadersBag). I think we should have header classes in which we can handle particular header use-cases (like zf2 headers) and if we do so, this improvement will make more sense.