Tile-Based game [Java2D] (128x128px). FPS drops when resizing the window!

Hello fellow coders :smiley:

I have a problem with my tile-based game.
I want to make a Tile-based game with 128x128px sprites. The problem is that I only have ± 50 FPS when I try to render my tiles (at a
high resolution). My code looks quite simple but I dont know why I only have 50 FPS.
Here my Main class: http://pastebin.com/gPTknRZt
Even if I remove the level.render(…) method I still have 50FPS.
EVEN if I only draw a black screen I only have 50FPS, so I have to do something terribly wrong.
Is it good to make the image like twice as big as the window and then scale it down and render it so you can fit more tiles on the window. Or how should I re-size my image without many frame drops.

Thank you for your help.
Any help is appreciated. 8)