Often perf reqs dictate the systems/languages/FWs that may be considered. Thus these reqs are very important to be known up front. You can add many features la

Often perf reqs dictate the systems/languages/FWs that may be considered. Thus these reqs are very important to be known up front. You can add many features later, but some level of perf is hard once you have 50k line of app code on top of, for instance, Rails. This because it's hard to change language/FW, compared to adding a new functionality on top.