The cache friendly alternative is to use a different data structure. There is no cache-friendly iterate-in-order on the standard map.

But I've got plenty of cases where this in fact is cache friendly, because the entire map fits into L2 or even L1 anyhow because it's going to have maybe 4 keys in its lifetime. Not every map has fifty million values in it. I'm always keeping at least a little bit of track about such details when I'm using maps.

I did some benchmarks, and it seems you are right that there's no (more) cache-friendly solution in general (at least, not that I could come up with). Memoizing the full entries (key and value) into a slice and then sorting that slice by key has basically the same cache-thrashing characteristics as randomly accessing the values, and is no faster (sometimes slower).