What is _app_start in firebase performance actually measuring?

Question

According to this documentation, the measurement of _app_start

• Starts when the app's FirebasePerfProvider ContentProvider completes its onCreate method.
• Stops when the first activity's onResume() method is called.

My app has two activities

• LoginActivity
• MainActivity

LoginActivity being the launcher activity.

In the onCreate method of LoginActivity, I check if the user is logged in and open MainActivity if true.

My expectation was that the measurement of _app_start should stop when the onResume of LoginActivity is called. (as this is the first activity)

However, I am having trouble understanding these logs (starting the app for a logged in user).

The first log shows that LoginActivity was rendered in 620ms, but the _experiment_app_start_ttid records 1988ms.

Additional Info: I am manually logging the app_start event in firebase analytics from MainActivity.

Following are my doubts

1. Why is the value of the Displayed log(620ms) different from _experiment_app_start_ttid (1988ms)? Is this trace something entirely different from _app_start?
2. How can I log the value for _app_start trace?
3. Does manually logging the app_start analytics event in MainActivity affect the _app_start trace measurement? (like is it stopping after the onResume of MainActivity?)