It is a pretty high QPS. I work on a chat service that is quite popular (even more popular than Slack) and looked at our QPS to our persistent data stores. It is less than an order of magnitude compared to slack's when you normalize for DAU. Really wonder what they're doing over there.
Whatever the service, it isn't the platform that Slack is. I would guess that a huge percentage of their db traffic is from apps and custom integrations.
