I haven't looked at the problem closely enough to answer, but could we start from the other direction: what makes you think that memory I/O would be the bottleneck?

From my limited understanding, we sequentially bring a large text file into L1 and then do a single read for each value. On most processors we can do two of these per cycle. The slow part will be bringing it into L1 from RAM, but sequential reads are pretty fast.

We then do some processing on each read. At a glance, maybe 4 cycles worth in this optimized version? Then we need to write the result somewhere, presumably with a random read (or two?) first. Is this the part you are thinking is going to be the I/O bottleneck?

I'm not saying it's obviously CPU limited, but it doesn't seem obvious that it wouldn't be.

Edit: I hadn't considered that you might have meant "disk I/O". As others have said, that's not really a factor here.