In that case, loop fusion may be even more efficient - makes better use of CPU caches. Depends on the logic I suppose.

That's the thing, it's feels impossible to generalize since this can get pretty arbitrary. The size of the data, how sorted the data already is, what you're processing, etc.

Yes heuristic logic can have a role here, e.g. if list has length M and number of passes through it M, do N*M and if the result is over some threshold then make the effort of sorting the list. But a good value for that threshold depends on many factors, hence a heuristic may make sense in some scenarios. It's the same sort of pattern used with SQL query optimizers, e.g. cardinality of a result set is one of the key factors that influence how some query on that result set will be executed, e.g. whether to scan versus use an index.