Performance Testing Versus Unit Testing

I'm reading Osherove's "The Art of Unit Testing," and though I've not yet seen him say anything about performance testing, two thoughts still cross my mind: - Performance tests generally can't be unit tests, because performance tests generally need to run for long periods of time. - Performance tests generally can't be unit tests, because performance issues too often manifest at an integration or system level (or at least the logic of a single unit test needed to re-create the performance of the integration environment would be too involved to be a unit test). Particularly for the first reason stated above, I doubt it makes sense for performance tests to be handled by a unit testing framework (such as NUnit). My question is: do my findings / leanings correspond with the thoughts of the community?