Serving static files is the native behavior of nginx, and when we use gateway for scripting, it needs more works by nginx. Thus, loading a php file should take more time than loading an image file.

I compare the speed of loading an image file with serving via php by

$image=imagecreatefromjpeg($imagepath);
header('Content-Type: image/jpeg');
imagejpeg($image);

Surprisingly (at least to me), the speed of loading the image via php was 50 - 70% faster than loading the original image file directly. What is the reason for this peculiar behavior?

I tested with Pingdom. The difference was in Receive step, which is associated with downloading the data.