Why does this simple Firestore 'where' query take so long?

Question

This query is taking longer than 10 minutes to execute and I can't figure out why. In case it's a factor, I'm running it when my app loads in react native's componentDidMount method.

There are about 12k docs in the users collection, but only about 30 of which have a fetchStatus of 206. It also seems to slow down my computer and/or internet connection while it's running.

Here's the code:

firebase.firestore().collection('users').where("fetchStatus","==",206).get()
.then(querySnap => {
    console.log(querySnap.size)
})

EDIT: It seems it's something to do with my React Native environment. When I run the query via a Cloud Function, via this code, it's nice and quick:

export const count206 = functions.https.onRequest(async (request, response) => {
    try {
        const querySnap = await db.collection('users').where("fetchStatus","==",206).get()
        console.log(querySnap.size)
        response.send("ok")
    } catch (error) {
        console.log(error)
        response.send(error.message)
    }
})