My advise is that, when a customer is looking for high performance, high usability and high security software within a less budget, they have to put things in priorities, in terms of requirement, data structure and Testing, they have to take this in consideration and the QoS.
As Mr. Daniel has said, this situation demands prioritizing. For sure, testing is demanded also for performance.
Regarding usability (or, more generally, HCI), I focus on consistency, based mainly on the referential Payne and Green's "Task-action grammars: a model of the mental representation of task languages".