When I added the MobileAdvert plug-in to the project and exported to Android apk, a black screen (about 10 seconds) would appear when the apk was opened on the mobile phone, and then finally the game content appeared.
Usually that means the ads are not configured, or the configuration is missing. If the ads are correctly configured it should start immediately. The only reason it takes a long time if it's misconfigured is that a bug in Google's code causes it to hang, and Construct works around that by waiting for a period of time and then continuing with ads disabled if it doesn't respond in time.