Thanks! We've found that this scales fairly well. Tables generated from the activity stream are generally done in seconds or minutes at the worst case.

One reason for this is that most warehouses are column-oriented. This means our table, which is 11 columns wide but pretty deep, is really fast to query.

We had a customer on a 3 Node (tiny) DS large Redshift warehouse dealing with billions of rows. Their Looker queries literally took 8 hours. We moved them to the Narrator structure and they saw the same queries assembled using the activity stream down to minutes (<5 minutes)

Edit: I should also point out that all queries in Narrator are compiled to SQL that runs directly on the warehouse. We don't use any external data stores or anything like that.