Performance improvement splitting huge file

[mdijkens]

Thanks SMcNeill, but this puts the first part of the inFile in the first outFile, etc.
1111222233334444 > 1111 2222 3333 4444

What I need (and requires a lot more processing) is:
1234123412341234 > 1111 2222 3333 4444

So every next byte (or record) needs to go the the next file...

I've really tried to narrow down the snippet to as small and simple as possilbe; please take a look what it does