Lazy parsing on a packet stream

[rick2047]

I am trying to parse a binary file format which is made up of packets. I have figured out the packet grammar, and if I have a file with the stream saved in it I can parse all packets from it. But this is incredibly slow as each file can be several GBs long. To speed it up I was wondering if I can only parse specific packets selected by their ID which is right at the top of the structure. A minimum example of a packet would look like this

MAGIC NUM
ID: ID1
DATA1: VALUE 1
DATA2: VALUE 2

I want to parse till ID and then decide if I want to parse any further.

One approach I tried was to write a header parser. But the problem is since I don't know the size of the packets, I end up writing a negative look ahead for the magic number. This is equally as slow as just parsing the struct (this does feel like one of those good problems doesn't it :P).