Honestly its a little surprising the first optimization he found was something fairly obvious just by using perf. I thought they had discussed the zeroing buffers issue in the first post? The second optimization was definitely more involved/interesting but was still pointed at by perf. Don't underestimate that tool!

AFAICS, it wasn't “just perf”; it was doing a differential profile between the C and Rust versions, with manual matching up. (perf diff exists, but can't match across the differing symbol names, and few people seem to use it.)

He came from the aarch64 perspective on an Apple device. I often experience someone spotting an "obvious in hindsight" gap because they come from a different background.