Firebase login with google much slower than native google login, JWT takes a long time

### Describe your environment

* Operating System version: OSX
* Browser version: chrome 76.0.3809.132
* Firebase SDK version: 6.4.1
* Firebase Product: auth

I've noticed that following the [login with google guide](https://firebase.google.com/docs/auth/web/google-signin) as mentioned in the firebase docs is significantly slower to load the actual google login page than using the [login with google](https://developers.google.com/identity/sign-in/web/sign-in) and then [passing the creds to firebase](https://firebase.google.com/docs/auth/web/google-signin#advanced:-handle-the-sign-in-flow-manually)


Notice that the firebase popup shows about:blank for a good 2 seconds.

I've made a minimal reproduction to demonstrate this, (i'm using angular but it doesn't matter)
**[https://itsteknas.github.io/fierbase-login-delay-reproduction/](https://itsteknas.github.io/fierbase-login-delay-reproduction/)**

This bug is for two reasons,

1. The time it takes to load the google login in firebase is too much compared to gauth ~3-4s
2. The time it takes to get the firebase JWT after successful gauth is again too much ~2 s

The problem is only worsened by the poor connectivity while using cellular network.
You can try the demo page with some network throttling and see it for yourself.