When the clock is ticking on a project, and deadlines are looming over developers like a long shadow, the first few items to go prior to launch are a familiar refrain: accessibility testing (esp if not a gov/edu site), and you guessed it - performance testing. The sad part is that they should both be baked into best practice workflows so that you've created a great base to start from. But most of the time, it isn't the case due to a myriad of motives and none really being acceptable.
In the case of performance you're face w/ a duel between the speed of the site versus it's speed to sales. Everyone from the sales department to the marketing team want to have something out to users, but the very sales they search for are proven to be at risk if performance isn't tuned.
Front end developer and author Jeremy Wagner discussed key steps you can and should consider baking into your workflow so that even though testing should be on going, some best practices can help with the shock and awe of poor, post push performance. You can head over to read the Performance Calendar Day 10 Speed versus Speed To Market by Jeremy Wagner for the full details.