Eurogamer has conducted a test of the PS3 and Xbox 360 versions of Mass Effect 3 and has come to the conclusion that problems with frames per second slowdown identified when the PS3 demo for the game was released still remain. The Xbox 360 version of the game runs at about 30 FPS, while the PS3 version runs at 20 FPS - the same as the demo. A side-by-side comparison tracks it in real-time, which you can see in the video embedded in this Eurogamer story.
"As the video demonstrates, the demo is highly representative of the final game on both platforms, with 30 frames per second being an unachievable target during some cut-scenes on Sony's console, particularly at points with multiple NPCs and transparent alpha effects on screen," notes the Eurogamer report. "The 360 version is also liable to dip during like-for-like circumstances, although never quite as much; as per the demo, the action remains v-synced, though the odd, unnoticeable tear may occur at the very top of the screen when the engine is under stress. As far as traversal gameplay goes, the 360 only shows occasional blips in visual feedback, as opposed to the more regular stretches of 20FPS gameplay we can see on PS3."
The report closes by saying that "frame rate dips impact controller response during shoot-outs to a certain extent."