Moon's Aa for windows phone, the frame rate is bad and unstable.

this game runs smoothly on Android and IOS, you can download them on google store and apple store. but the framerate is bad on windows phone ( Lumia 640). I see the fps is 60 on title screen and it drops down to 35~45 after enter game screen. I have disabled all the texts. Can anybody give some ideas?

My Dev Environment:

windows 8.1

visual studio 2013 community

corona card: #2662

Lumia 640: windows phone 8.1

here is the link of windows phone project.

https://www.dropbox.com/s/ebjuc1u9xoskr9y/CoronaCardsApp1.zip?dl=0

thanks a lot!