Thanks for very useful description, Maxim!
Actually, I suspected the nature of 0.000 request time; but what makes me wonder is that it takes only 3ms to process a relatively php file with output of 5KB. Then, why reading a static file of 15KB should take 600ms? If it is normally the case, isn't it better to save images as data instead of file? PHP should process a base64 string faster.