If you can't gain traction in a market due to lost conversions, you'll never have a chance to optimize it.

Not to mention that some infrastructure choices make it really hard to optimize later, if not impossible (sans re-writes).

Also worth noting: the phrase "Premature optimization is the root of all evil" had a very different connotation when first uttered:

http://ubiquity.acm.org/article.cfm?id=1513451