Sometimes it's overrated. Sometimes poor performance is due to bad design, or worse, premature optimization...

What's worse, sometimes you write a nice, pretty, and slow implemenation that actually incurs a business cost, and then you have to do more work (or buy new hardware) to fix it.

By then, the whole thing is probably into production, so rewriting the whole thing will be frowned upon, especially when you just wrote it in the first place. In general, due to business politics, you get one chance to get a system right every five to ten years; after that, you have to support it.

Do it right the first time. Otherwise, you'll end up doing it right five years from now... and be yelled at for five years for writing a slow, clunky system that you're not allowed to fix. :-(