When we speak of performance, we generally speak of the capability of an application to perform fast enough for our needs. That means different things to different people, but it's safe to say that if your application doesn't do what your user needs in a reasonable amount of time, it doesn't perform well. In the case of a commercial application or site, this could be a disaster because your audience might find somewhere else to go, and that could cost you money.
Scalability is the capability of your application and systems to perform well as their user count grows. Chances are that a couple of sloppy database queries every few seconds will not bring your system to its knees, but if suddenly your user base climbs into the tens of thousands, and the application can't handle it, it doesn't scale.