Performance

Comparing the execution time of FirebaseApp (Rhino JS) vs Firefast (V8 Runtime)

Firefast promise is 2x faster than FirebaseApp, when you are making 10+ requests. Firefast async/await is 4x faster than FirebaseApp, when you are making 10+ requests.

Use async/await whenever possible

In theory, promise shouldn't be slower than async/await, since it executes the same Firefast code. But, for some reason (probably due to Apps Script implementation) it is significantly slower than async/await. So, use async/await whenever possible.