[exim] how does exim get $header_XXX variables?

Does it scan the header file everytime it needs a specific $header_XXX?

Lets say I need to check $header_X-SPAM-RESULT in many different
routers, is it better if I put it in a $address_data? Or a "call" to
$header_XXX is fast enough?

BTW... does $address_data stays just in memory? I assume so, right?

Thanks a bunch!