One of the reasons I’m drawn to the performance side of the business is that it’s one of the few pieces of of your site or application where you can really know when you’re doing the right thing.
There are a lot of questions that can run through your head during a site build. Is the design right? How about the feature set? do these labels make sense? Is the language correct for my audience? Does this visualization truly illustrate the underlying data and add meaning? Was I right to use canvas? Should we be using web storage?
Those (and many other examples) are the things that can (and should) your thoughts. You can leverage experience and best practices throughout, but there are always mistakes, miscommunications and surprises waiting around the corner, so sweating the details of a design or interaction model can make or break an application.
One aspect that brings a little bit of certainty is the question of speed.
The site is either fast, or it isn’t.
There are certainly degrees of “fast,” a full featured application won’t be as fast as Google.com, but once you’ve defined a speed goal there are plenty of ways to know if you’re hitting your marks which makes it a bit more satisfying than the stuff that’s a little more nebulous (or takes a little longer) to measure.