Developers or clever peoples WANTED

Hello I'm in the middle (actually in the end) of developing a game for WP8. FYI I use GameMaker Studio. Well to the problem. I have a Lumia 920 and have therefore tried to "tailor" for the Puremotion HD+ screen by using 60 steps (frames per second) and 1280x768 resolution. Until today I thought it ran fine, it could be a bit more smooth, but it was acceptable IMO. Back to the point my sister recieved a Lumia 820 today and I was really excited to test it on another resolution without changing the code. The result was very suprising for the newbie dev I am. The game scaled perfectly. I can't see it's not made for 800x460 however the suprising part is it's so much smoother! There is no lag at all and the game runs alot faster (I actually have to turn down the speed of the objects). So how can this be? The Puremotion HD+ is supposed not to lag and it's the excact opposit. I then tried to lower the steps to 30 and double the speed of the objects but then it just looked laggy on both of the devices (the 820 still looked better). Do any of you have any advice for me? How can this be? The last thing I think I could do is to lower the graphics to 800x460 but that will take so long and I want to avoid it.

TL;DR

My sister's 820 smokes my 920 when playing my homemade game. Help me.

As english is not my first language it can be a little difficult to read. Please feel free to ask me questions if you want to know something.