you should call the loadAd at some point like starting a new level.
rename the AdView1_ReceiveAd to showInterstitial and call it when you display a gameover or level completed screen.
like it is now you start loading an ad when you actually want to display one and that causes the delays since it still need to fetch one.
you don't have any control on when that ad will show up and that's why they (Google) nag about it.