Gender as a performance is the concept that there are no inherent traits to one's gender; we only act a particular gender, subconsciously in our daily lives, in order to be more easily identifiable to society. By wanting to fit in, we see how role models and our peers conduct themselves and act accordingly. Yet even as we do this, our peers are also watching us for the same clues, resulting in a reinforcing cycle that defines what it means to be a particular gender. As such, it is easy to assume certain characteristics of a person, whether accurate or not, based purely on identifying their gender.
An even bigger point of concern occurring from this is that this necessitates that most, if not all, actions/performances are to be are divided between the two sexes. Adding in that these actions have values placed on them, a system where one sex or gender could itself been seen as more valuable to society. Redefining the gender roles within society would be an effective way of remedying this should it be a problem, but given the positive feedback on the system, it would be a daunting task.