When you are working on commercial software or even just industrial strength business software you have to balance things. Time to market is everything and while you must have usable, quality code, you have to get it done.
One of the things that is hardest to balance is performance tuning, it is often best to put the lion’s share on hold until version 1.x or even 2.0.
Most software companies to not invest heavily in performance tuning in the first version of a product since it usually requires alot of work and often customer reactions will force you to make changes that might invalidate the tuning anyways.
You should certainly consider doing serious performance tuning if the feedback from QA is that your software is too slow under load to give to a customer, but that is just common sense.