A range of 1 billion can only solve efficiently an up to maybe a 59-bits private key of a known public key, if this is what you are trying to do. But I think you got the strategy wrong - the target point needs to get subtracted in sqrt(rangeSize) 1 billion delta increments and then looked up in your "database" (which should actually be used with a bloom filter first, and definitely not as 65536 text files).

A range of 1 billion can only solve an up to maybe a 59-bits private key of a known public key, if this is what you are trying to do. But I think you got the strategy wrong - the target point needs to get subtracted in sqrt(rangeSize) delta increments and then looked up in your "database" (which should actually be used with a bloom filter first, and definitely not as 65536 text files).