Maybe it's my video card, but Jnes (along with many other Windows-based emulators) runs very slowly when I try to expand the screen to full screen, or even to simply twice the size of the smallest window. Any suggestions?
There's a great many reasons, but your going to have to specify your hardware. (Processor and speed, video card type plus video memory, Ram, and operating system).
My old computer (450MHz) could not handle a stetched NES image under JNes because I didn't have the speed nor the graphics card to do it. That's why I bought a new PC (partly)