Stream fusion is a fancier version of the concept of loop fusion: https://en.wikipedia.org/wiki/Loop_fission_and_fusion

At a high level and skipping details, suppose you want to do something like this:

  (filter predicate (map f sequence))

Naively, this is two linear passes over the data. Fusion would allow you to combine these into one single linear pass.

This has particular uses when there are intermediate, temporary allocations that can be eliminated, it's not just about time performance.

> Come on frontend people, just stop. Please, just... stop.

I suspect this is part of why you're getting downvoted. This site seems to be a bit of a gag in its presentation. It's over-the-top on purpose.