[benc666]

"Premature optimization is the root of all evil" means: make it work correctly first, and then - if anybody cares - make it fast. Readability/maintainability, then only as needed trade it off for speed, because not all code needs to be highly performant.

[api]

Mercurial works correctly but is unbelievably slow compared to git.

If you don't think about performance ahead of time (preferably at the algorithm level) then you will build something correct but slow and it'll be very hard to make it fast without redesigning major parts of it. It's possible to make design choices that paint you into a performance corner very easily, especially with something designed to work on a lot of data. O(n^2) might look like O(log n) for small data and in testing.

[Shorel]

From the Knuth paper:

"We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we should not pass up our opportunities in that critical 3%."

It means make it fast in the code that matters, do not try to make it fast in every single line.

To say the quote is often misunderstood is the biggest understatement in modern computer science.