What really grinds my gears.
When a co-worker gives me feedback from a review of my code be like "you should use if..elseif..else instead of switch, because it performs much better.
in this case performing much better means it is only 25 pikosecond instead of 30 pikosecond in a function which is called apprx. 3 times a month in production *facepalm*

I'm always with "let's get maximum performance from our software", but c'mon... really?!???