Performance is the elusive butterfly of API development. Everybody is intrigued with its beauty, yet few know how to capture it.
In the old days, the approach of many shops to ensure a performant API was to create some code and then pass it over to the wall to QA to do load testing. Later some integration testing took place. As long as the API worked and it was met some marginal performance benchmarks, things were good.
This worked well when a public, HTTP based API, consumed by a wide variety of distributed devices was more the exception than the rule. However, today APIs are a big deal and they are everywhere, so much so that companies are posting very big infographics prominently in the front page of the New York Times to create even more awareness about the technology to the general public.