[MindSpunk]

Doing 2 loops over the data means you have to do a full pass over the data twice. If your N doesn’t fit in L3 then you’re going to load N twice instead of once. Loading twice, even out of L1 is still slower than never loading twice at all.

[moron4hire]

Exactly. And there's also the fact that sometimes the data stream we're processing is unbounded and ephemeral. For example, reading values from a physical sensor. It may not match up to this specific example, but the point remains that a single "loop" over a data set might be all you get, so pack as much into that loop as you can.