Windows Phone 8.1 game with CocosSharp 1.4 or 1.5 is very slow

I experience slowness in my game. Also, the same slowness can be seen in CocosSharp sample called Angry Ninjas https://github.com/mono/cocos-sharp-samples/tree/master/AngryNinjas/src
Run it on WP8.1 device and see how slow the game is.

My game runs smoothly on iOS and Android but is very slow on WP8.1

Please advise.