Firestore: slow query - firebase

Question

I have a query that gets all the messages from my messages collection for a particular chatId.
It then sorts it in descending order on the createdAt timestamp and has a limit of 20 messages applied to that.. My queries take ~2500 ms... which I feel is slow.. I don't even have many messages in my collection so I feel like it could get slower once I have thousands of them..
I'm new to firestore, so perhaps I'm doing something wrong? I appreciate the help!
Here is the code for the query:
exports.getMessages = functions.https.onCall(async (data, context) => {
try {
const startAt = data.startAt;
const increment = 20;
const limit = startAt + increment;
const chatId = data.chatId;
const snapshot = await firestore
.collection('messages')
.where('chatId', '==', chatId)
.orderBy('createdAt', 'desc')
.limit(limit)
.get();
const promises = [];
snapshot.forEach(doc => {
const message = {id: doc.id, ...doc.data()};
const promise = combineUserAndMessage({
message,
userId: message.user._id,
});
promises.push(promise);
});
const messages = await Promise.all(promises);
return {data: messages, startAt: limit};
} catch (err) {
console.log('GET MESSAGES ERROR: ', err);
throw err;
}
});
const combineUserAndMessage = async ({userId, message}) => {
try {
const userSnapshot = await firestore.doc(`users/${userId}`).get();
const user = userSnapshot.data();
const createdAt = new admin.firestore.Timestamp(
message.createdAt._seconds,
message.createdAt._nanoseconds,
).toDate();
return {
...message,
user: {
_id: userId,
name: user.displayName,
avatar: user.profilePicture,
},
createdAt: createdAt.toString(),
};
} catch (err) {
console.log('COMBINE USER AND MESSAGE ERROR: ', err);
throw err;
}
};
EDIT: I took the benchmark from the function logs. I understand that there can be cold start costs included in that benchmark so I'll try to benchmark it using my own logs. That said, if anyone sees something wrong with the query, or knows of a way to make it faster/better, I'd still appreciate it!