Premature optimization makes it harder to write fast and efficient code. That is because you guessed where the bottlenecks would be before you had enough inform

Premature optimization makes it harder to write fast and efficient code. That is because you guessed where the bottlenecks would be before you had enough information (which is why it was "premature"). The optimizations make the code more brittle and hard to change so that when you reach a position to measure where the bottlenecks are it is too complicated to do the proper optimizations.