> pretty much every time I've seen a developer complain that something is slow and then 'prove' that it can be faster by making a proof-of-concept the only reason theirs is faster is because it doesn't implement the important-but-slow bits and it ignores most of the edge cases

It's still a good place to start a discussion though. In such a case, apparently someone believes strongly that things can be made much faster, and now you can either learn from that person or explain to them what edge cases they are missing.