When I think of "fast", I always think of a time I was building a product that required subscribing to Twitter's API to receive updates from specific users.

In my testing, I'd set a breakpoint on my server code to see when I'd get the "push" from Twitter's API and would use the Twitter App on my phone to create test tweets. Every single time, I'd create a tweet and my server breakpoint would be hit immediately. Not soon, but immediately. I'd see my breakpoint triggered well before the UI in the app even refreshed after submitting the tweet.