FPS Issues

Hey guys,

Have a Lenovo Y40 here, with and i7-4510u and an amd r9 m275. Before you guys begin to shout at me for having a laptop for gaming, i just want the answer to one question. I was getting 100 fps + on all high settings. I found my dad's old monitor (1680x1050), and since the y40 didnt have a vga port, i started to use my converter. However, i'm getting lower fps on my monitor, which is lower resolution than my laptop sscreen itself. Why is this happening and how should I deal with the problem?