I think Melee has a well known issue (in the Melee community at least?) of characters falling faster than they should. At least this seemed to be the case the last time I played it, dunno if it has been taken care of recently.
As for other games, I haven't noticed anything aside from Melee exhibiting and sort of unnecessary speed-ups. Have you checked to make sure that your game speed (Dolphin represents this as a %) is more or less 100% all the time? A sudden spike might explain what you're experiencing. Generally though, when Dolphin says a game is running at 100% it's as close to matching the Wii's speed as anything non-Wii/Wii-U will get.
As for other games, I haven't noticed anything aside from Melee exhibiting and sort of unnecessary speed-ups. Have you checked to make sure that your game speed (Dolphin represents this as a %) is more or less 100% all the time? A sudden spike might explain what you're experiencing. Generally though, when Dolphin says a game is running at 100% it's as close to matching the Wii's speed as anything non-Wii/Wii-U will get.