I don't think there is a strict rule to classify. But usually a performance polymer will have at least one very exceptional property. This could be its hardness, wear resistance, temperature window, load bearing capacity etc.
In most cases these are more expensive per kg than a run of the mill commodity polymer. Oftentimes production volumes are much smaller than the millions of tons of a commodity polymer.
The materials are often blends sometimes of a polymer & a non polymeric additive. Frequently tight specs. on MW or polydispersity etc. is possible.
Market differentiation is higher; company A and company B might make a substantially different product. For some large users batch control might give a designer polymer that matches its optimal application properties very precisely.