加载插页式广告时，如何减少延迟时间？

Question

我有3个tab（片段）android应用。我添加了在点击“搜索”片段时打开的插页式广告，但是广告需要花费一些时间才能加载。如何减少广告加载时间？我想在用户点击“搜索”标签（片段）后立即弹出广告。

代码如下：

public class MainActivity extends AppCompatActivity {                                                                                                        

public InterstitialAd mInterstitialAd;                                                                                                                  

@Override                                                                                                                                                
protected void onCreate(Bundle savedInstanceState) {                                                                                                     
    super.onCreate(savedInstanceState);                                                                                                                  
    setContentView(R.layout.activity_main);                                                                                                              

    loadInterstitial();                                                                                                                                  

    BottomNavigationView bottomNav = findViewById(R.id.bottom_navigation);                                                                               
    getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,                                                                      
            new HomeFragment()).commit();                                                                                                                
    bottomNav.setOnNavigationItemSelectedListener(navListener);                                                                                          
}                                                                                                                                                        


private BottomNavigationView.OnNavigationItemSelectedListener navListener = new BottomNavigationView.OnNavigationItemSelectedListener() {                
    @Override                                                                                                                                            
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {                                                                                    
        Fragment selectedFragment = null;                                                                                                                
        switch (item.getItemId()) {                                                                                                                      
            case R.id.nav_home:                                                                                                                          
                selectedFragment = new HomeFragment();                                                                                                   
                break;                                                                                                                                   
            case R.id.nav_favorites:                                                                                                                     
                selectedFragment = new FavoritesFragment();                                                                                              
                break;                                                                                                                                   
            case R.id.nav_search:                                                                                                                        
                selectedFragment = new SearchFragment();                                                                                                 
                displayInterstitial();                                                                                                              
                break;                                                                                                                                   
        }                                                                                                                                                
        getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,                                                                  
                selectedFragment).commit();                                                                                                              
        return true;                                                                                                                                     
    }                                                                                                                                                    
};                                                                                                                                                       

private void loadInterstitial() {                                                                                                                        
    mInterstitialAd = new InterstitialAd(this);                                                                                                          
    mInterstitialAd.setAdUnitId("ca-app-pub-3940256099942544/1033173712");                                                                               

    AdRequest adRequest1 = new AdRequest.Builder().build();                                                                                              
    mInterstitialAd.loadAd(adRequest1);                                                                                                                  
    mInterstitialAd.setAdListener(new AdListener() {                                                                                                     
        @Override                                                                                                                                        
        public void onAdClosed() {                                                                                                                       
            mInterstitialAd.loadAd(new AdRequest.Builder().build());                                                                                     
        }                                                                                                                                                
    });                                                                                                                                                  
}                                                                                                                                                        

private void displayInterstitial() {                                                                                                                     
    if (mInterstitialAd.isLoaded()) {                                                                                                                    
        mInterstitialAd.show();                                                                                                                          
    }                                                                                                                                                    
}                                                                                                                                                     

}