Fps drop on higher resolution

Hey there guys,
after several hours trying to fix this issue, i really need some help.
The biggest problem is that the game runs smooth, with constant 60 fps in a ~1024×768 resolution. But the higher i get somthing causes a drop to about 20 fps. I started realizing it after packaging to a 1280x720-W where sometimes my charcaters move in a slower way the normally should. After trying downgrading some particles( well we are talking about 2 constant particles and 2 or 3 adding; so about 5 max at one time) and packaging it to fullscreen its totally unplayable (20-25 fps constant).
Aswell i have to say the drop even occurs when there is nothing happening…at least on screen.

Perhaps there is somebody who can explain me in which way i have to improve or to decrease data to have a flowing game. Btw its all done by blueprints (hopfully not that big of an impact).

I really appreciate every help and i will try to give any information you need to know for better understanding.
Best regards P.A.

Get a better PC that can handle it, if you are using that resolution, that is bad, and full screening it is probably making it 1080p.

well i think this is not the problem…
my system information:
Intel core i7 cpu 2.60 GHz
16 gb RAM
GeForce GTX 960M

The Game has no such high poly or insane Graphics as you think…its not a Crysis game.

Add me on skype for help. cjsavage56