I use Firestore via flutter but never observe such slowness. I want to know whether the problem is specific to your app or Firestore.
Do you want to share a minimum reproducible project and Youtube video to show the slowness?
There is a chance that your app is downloading unnecessary data, which results in charging you unexpected money. You want to get the bottom of the slowness. Anyway, looking forward to hear from you.
We are developing an flutter app (Pixz:- android & iOS) and everything works fine except the firestore queries. When we launch the app for the first time everything is smooth and run fine but after a while firestore queries jams the app. Simple save query takes 5 minutes to execute.