[shrink]

After watching network requests, I think it's based on the use of the Javascript login functionality vs. the redirect functionality.

If the "Login with Google" button opens in a new tab and the Google OAuth flow completes in the second tab, then the process will have the "loading" screen after clicking "continue" because "loading" indicates Google OAuth is communicating back to the original tab. If the "Login with Google" button opens in the same tab, clicking "continue" triggers a 302 redirect to your callback URL of which the loading speed is controlled by your website.

The immediate workaround is to switch to opening the Google OAuth login page in a new window.

edit: "Sign In with Google for Web" appears to be what provides the new tab for login functionality https://developers.google.com/identity/gsi/web/guides/overvi...

edit edit: that's not to say you're wrong, Google should definitely fix this but "Sign In with Google for Web" is not impacted in case anyone needs an immediate fix for their own apps, they can switch to "Sign In with Google for Web" (a difference user interface for OAuth).

[modeless]

The problem with this is it doesn't support OAuth scopes. So if you need any Google account permissions beyond a simple sign in you can't use it.