Where should I be "merging" Firebase Auth and Firebase Firestore user data?

I have a NextJS web app where a user can login and perform the typical e-commerce tasks such as editing their emails, name, preferences, etc.

Currently a user can login using their email or Google account as outlined here: https://firebase.google.com/docs/auth/web/firebaseui

There is a step I have on the account creation after the user successfully authenticates through email or Google account, the code will look for an existing user object in the Firestore to see if an existing account already exists.

This works fine for now but I notice some slowness when a new user creates their account and I suspect the cause is this check that’s being done.

This is my first e-commerce website and I’d like to know what the optimal way to do this. I was thinking of moving this action of checking to a Next API middleware but I figure I’d ask here first if that’s worth doing.

Below is the code for the login page.

... 
export default function Login() {
  const router: NextRouter = useRouter()
  const user = useAuthContext()

  const { profile, setProfile } = useProfileContext()

  const uiConfig = {
    signInOptions: [
      GoogleAuthProvider.PROVIDER_ID,
      EmailAuthProvider.PROVIDER_ID,
    ],
    signInSuccessUrl: "/account",  # Reroute to user account dashboard 
  }

  useMemo(() => {
    if (user && !profile) {
      uidProfileExists(user.uid)
        .then((exists: boolean) => {
          if (!exists) {
            // This will create a new profile as recognized by the Firebase
            // database.  A user login profile may already exist through
            // Gmail, GitHub, or some authentication provider.
            createNewUserProfile(user)
          }
        })
        .then(() =>
          getUserProfile(user.uid).then((profileData) => setProfile(profileData))
        )
    }
  }, [])
...