Performance(speed) issue during login session

I am using a simple address book app with data sheet that has about 250 lines.
In addition, I manage user-login system by email whitelist.
However, after creating and using multiple apps, I found that it takes more than 5 seconds to log in through email authentication.
Is there any way to speed up the time taken to log in?
For your information, I am developing and using this app in South Korea and didn’t upgrade to pro plan.

You can’t use the email whitelist style login if it’s not a Pro app. So not sure what could be happening.