You can safely ignore this one if you are working on server software. Instead you really should focus entirely on functionality and ignore all considerations of performance until you actually have a problem. Be a dogmatic anti-premature-optimizer! You can always throw some hardware at the problem later.
If you are building something that will run on machines you don't control... well, then your problem is a bit different. If you are doing that and it's a something like a game, which must "feel right", well... maybe you should make some decisions up front trading off productivity for runtime performance. But even in those extreme cases, you are probably better off fixing speed problems when they come up, not before.
If you are building something that will run on machines you don't control... well, then your problem is a bit different. If you are doing that and it's a something like a game, which must "feel right", well... maybe you should make some decisions up front trading off productivity for runtime performance. But even in those extreme cases, you are probably better off fixing speed problems when they come up, not before.