That’s a great lesson to have early in your career, too.
My favorite example of that was a memory-bound app where a key function had been implemented in both C and x86 assembly. Someone tested and found that the assembler version had been slight performance loss since (IIRC) after the Pentium Pro, and since nobody had wanted to substantially change the assembly code going C-only meant they actually refactored the data access patterns and saw a substantial improvement.
My favorite example of that was a memory-bound app where a key function had been implemented in both C and x86 assembly. Someone tested and found that the assembler version had been slight performance loss since (IIRC) after the Pentium Pro, and since nobody had wanted to substantially change the assembly code going C-only meant they actually refactored the data access patterns and saw a substantial improvement.