Yes, of course you are absolutely right about the overhead of Bio modules and I also tend to use a simple parser myself if I know that this is only ever going to read from FASTA files. I'm just saying that trying to implement a FASTA parser yourself can be more tricky for the beginner than one might expect and trip you in unexpected ways.
Anyway, I think in this case it is the comparison of sequences rather than the parsing that is the main bottleneck.