It is, 'cause NTSC has 30fps and PAL 25fps (fps = Frames per Second)
So let's say you play PacMan and he needs 100 frames to move from the left side of the labyrinth to the right side.
NTSC: 100 frames / 30 fps = 3,33 seconds
PAL: 100 frames / 25fps = 4 seconds
You see, the speed is slightly faster on NTSC.
Good companies change the running speed of PAL games to compensate this problem, but most were just too lazy so most PAL games run too slow.