As someone with a Koi pond I got excited because this is a real issue with bio-load so I went into this thinking there might be info on how to determine food, fish quantity, etc. I shouldn't have been surprised what this actually is.

Capacity planning is not easy, it usually involves of understanding user journey and estimating the capacity needed from each subsystem from defined journey. What interesting in this article is the way they simulate the journey on the load test, they created "mini" slack client and use that to simulate the traffic. Even tho they have shared the situation, I think they still able to simulate it by using usual api hit based tools and that would be much easier to maintain.

Amazing to think they were actually spawning 150k headless browsers to simulate the traffic. That sounds like throwing money at the problem and it probably worked (for a while anyway).

Having built a load-test tool as well, I can say making it realistic enough and keeping it that way is possibly the hardest challenge. Maintenance cost is high, especially in a features focused environment.