iOS app gets slow and eventually crashes with Admob interstitial ads

After a while playing my iPhone game the phone starts to get really hot, and a while after that the app suddenly gets really slow and laggy, before eventually crashing.

I have followed official docs when implementing Admob ads.

I know for a fact the problem is Admob, since the app functions perfectly when turning off ads.

I know there is quite a lot of ads showing if you play for a while, but limiting the amount of ads displaying will only delay the lag and crash, and will not fix the underlying issue.

This is the code I use to load and display the ads:

- (void)viewDidLoad {
    [super viewDidLoad];
    [self loadInterstitial];
}

- (void)loadInterstitial {
    [GADInterstitialAd loadWithAdUnitID:@"ca-app-pub-xxx/xxx" request:[GADRequest request] completionHandler:^(GADInterstitialAd *ad, NSError *error) {
        if (error) {
            NSLog(@"Failed to load interstitial ad with error: %@", [error localizedDescription]);
            return;
        }
        self.interstitial.fullScreenContentDelegate = nil;
        self.interstitial = ad;
        self.interstitial.fullScreenContentDelegate = self;
    }];
}

- (void)viewDidAppear:(BOOL)animated {

    adCount = [[NSUserDefaults standardUserDefaults] integerForKey:@"adCount"] + 1;
    [[NSUserDefaults standardUserDefaults] setInteger:adCount forKey:@"adCount"];
    [[NSUserDefaults standardUserDefaults] synchronize];

    if (adCount > 11) {
        [self displayInterstitial];
    }
}

- (void)displayInterstitial {
    if (self.interstitial) {
        [self.interstitial presentFromRootViewController:self];
        adCount = 0;
        [[NSUserDefaults standardUserDefaults] setInteger:adCount forKey:@"adCount"];
        [[NSUserDefaults standardUserDefaults] synchronize];
    } else {
        NSLog(@"Ad wasn't ready");
    }
}

- (void)adDidDismissFullScreenContent:(nonnull id<GADFullScreenPresentingAd>)ad {
    [self loadInterstitial];
}

Nothing special. To me it seems like the memory gets overloaded or too many leaks after displaying enough interstitial ads.