The fact that all computational problems have a best case complexity bound and there are generally diminishing marginal returns as algorithms approach that bound (i.e. lower hanging fruit are found first). E.g. no amount of intelligence is going to find an algorithm that can sort an array of any arbitrary Comparable type on a single CPU thread faster than O(n*log(n)). There's room for improvement in better adapting algorithms to cache hierarchy etc., but there's only a fixed amount of improvement that can be gained from that.