Framerate issue with Project 64

I know its been a while since anyone has played with a 64 emulator but im having some issues. I downloaded the emulator and played mario 64 on it and it worked fine. But if i play cocnkers bad fur day or super smash brothers the frame rate goes from around 60 to 30. It makes it impossible to play. I have the standard plugins, i tried adding plugins but it gives me an error message and it also doesnt show it as an option. My gateway plays psx roms fine so i know its not the hardware. If someone can help me out i would appreciate it. Im thinking i might need a new graphic plugin but as i mentioned earlier i have no idea how to add it in the plugin file.