How secure is file header encryption for large files? I'm having trouble location information about whether or not encrypting a file's header is good enough. The files in question are media files that range in size from 500mb to 4GB. 

If I had no performance or time constraints, I would simply encrypt the entirety with AES256. However, with my current implementation (in C#), it takes about 3 minutes to decrypt a 2GB file. This is about 2m45s beyond my time limits. 

How vulnerable is that file's contents without the file header?

Apologies if this question is silly, this is my first foray into file security.
It depends on the format. The size suggests it is a video. However, video formats seems to be designed to be seekable. That means, the attacker seems to be able to:


Create some header (guessing the resolution and some other information)
If the beginning was damaged, play and seek in the file.