There are two sides to this. While on one hand, an update could decrease the performance of your device, on the other hand it will usually increase its security. Android 4.2.2 was released in 2013. 5.1.1 was released in 2015. Also many apps nowadays won’t be available for Android 4.X anymore.
It might. We are not sure why the app doesn’t work on your device. We can’t reproduce the issue. Chances are that the issue is the old Android version.