Well, I found out the mystery behind this question. First clue was
file_get_contents was very fast. Even if I'm using SSD, it shouldn't be that fast. So I dug around whole night and found some interesting information.
It's because file_get_contents is also returning the cached information. PHP itself doesn't include caching, but linux system has built in file cache which makes it extremely fast to access data repetitively.
Reference: page cache