Questions regarding FullScreenContentCallback

I used to start a new activity using adlistener on an interstitial ad like below.

showInterstitial();
mInterstitialAd.setAdListener(new AdListener() {
@Override
public void onAdClosed() {
startNextLevel();
}
});

Since RewardedAd and InterstitialAd APIs have been updated to align with all full-screen formats(https://developers.google.com/admob/android/migration), I changed my code like below to use FullScreenContentCallback method instead of adlistener.

mInterstitialAd.setFullScreenContentCallback(new FullScreenContentCallback(){
@Override
public void onAdDismissedFullScreenContent() {
Log.d(TAG, "Ad was dismissed.");
startNextLevel();
}

However, when using FullScreenContentCallback method on interstitial ad seems to have a lag. Whenever I close the interstitial ad, it goes back to the original activity, stops for a second, and starts a new activity.

This did not happen when I used adlistener; using adlistener, new activity started immediately after I closed the interstitial ad.

I experimented a little, and tried starting a new activity using rewarded ad's FullScreenContentCallback method like below.

mRewardedAd.setFullScreenContentCallback(new FullScreenContentCallback() {
@Override
public void onAdDismissedFullScreenContent() {
// Called when ad is dismissed.
// Don't forget to set the ad reference to null so you
// don't show the ad a second time.
Log.d(TAG, "Ad was dismissed.");
startNewActivity();
}
});

And it worked perfectly.

So my question is why does using FullScreenContentCallback on interstitial ad create a lag?

Is there a difference between FullScreenContentCallback on rewarded ad and an interstitial ad?

Is anybody else experiencing this delay?