I thought that using fork might speed up my search.

It's pretty unlikely, as it is. Can you split the file into two pieces and put each on separate hard drives with separate controllers? That's more likely to help. (It's not easy to say for sure. There's some black magic with regard to performance optimization. It's absolutely necessary to identify the single most likely bottleneck, though. That's IO here, as it usually is, and having only one file means that that's the most likely place for the bottleneck to start.)

If your actual code is about as simple as that which you posted, GNU grep might be more useful; see the -n option.