Games lag through playtime using Emulation Station

I'm currently using a Raspberry Pi Model B, and having some trouble. After starting most games, in this case we tried Mega Man X, we found that there is a meaningful amount of lag, causing the game to really slow down. The more games I put onto the memory, the slower that the added games get, but games that were already on the system, i.e. Doom, run fine, and older and less powerful games like the first Zelda mostly run fine, where Zelda crashes when you pause. I tried Earthbound later and found that it works with minimal lag, but when entering fights, skipping can be heard with the music and sounds. My system is a Raspberry Pi Model B with an 8gb SD card, an Emerson LF320EM4F 32-Inch Led 720P 60Hz HDTV, a four-port USB hud, a wired Xbox controller, and a wireless keyboard. Any help is appreciated.

Submitted by Dex33u Xbox 720 Release Date 2015