mp3 files preloaded completely

I'm using moc to play music in a directory I mounted with sshfs over the Internet.
It's working great with flac files, they are played very smoothly. But when playing mp3 files, there is a rather long delay before playback is starting, because moc is first reading (i.e. transferring) the complete file (while flacs are "streamed" on playback).
Is there a technical reason that mp3 files have to be read completely?

I also observed that when I enable ShowTime, it first wants to read ALL files I've added to the playlist (which makes some sense that the files have to be read to determine the time; though with flacs it's also working by reading just a small part of the file which is therefore rather fast).
But I don't understand why mp3 files have be read fully even when ShowTime is disabled.
Is it possible to optimize that? :)