Wouldn't a slower tick make it easier as you get more wall time to do the same challenge.

No? Wall time (that the challenge runs on) is unchanged, game time (Vsync) is running at 83% of full speed (50Hz vs 60Hz), so if something tied to frame rate (animation, walking speed etc.) takes 1 second to do on NTSC, it'll take 1.2 seconds to do on PAL etc.