System architecture is largely the domain of so-called "non-functional" requirements. These are requirements that describe what qualities a system should exhibit while it is meeting the functional dependencies that are its purpose. Often a system that meets the functional dependencies but does so too slowly, or using too many resources, or in a way that users find difficult, or otherwise fails to exhibit these qualities, is of little value. A large part of the architect's role is to have a care that the intended solution will exhibit these qualities.
Like any other kind of requirement, we seek to write these qualities down. This is difficult and too often vague adjectives are allowed to stand as statements of intent for a new system: "flexible", "maintainable" and the rest.
In every case (yes even "usable", with effort) the phenomena can be quantified, measured, and thresholds set. If this is not done, then there can be no basis for acceptance of the system by its users, no guidance for its builders as they work, no vision for those architecting it.
The questions to ask are simple. They include: how many? in what period? how often? how soon? increasing or decreasing? at what rate? If these cannot be answered then the need which the system is being built to address is not understood.
In many cases, the answers to these questions should be in the business case for the system being proposed and if they are not, then some hard thinking needs to be done.
The criteria must always be given as a range: the least possibly acceptable, the nominal, the most conceivable. If this range cannot be given, then the required system behavior is not understood. As the architecture of the system unfolds, it can be checked against these criteria to see if it is (still) in tolerance. As the performance against some criteria drifts, valuable feedback about the architecture is obtained.
Putting these ranges in place, and checking against them, is a time-consuming and expensive business. If no-one cares enough about the system being "performant" to pay for actual performance trials, then there is a good chance that it doesn't need to be after all.
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