Because unlike the good sis Katy, Gaga was dancing. Also she had that heavy ass outfit on his little ass frame, of course she was going to get tired. I suggest you lookup her audio vocals at the 2013 VMAs and see how much backing track help her when she keeps it simple.
Well, her "dancing" was hardly worth the lack of vocals. Nobody forced her to wear that heavy outfit. Regardless of excuses, it's a valid example. Sorry!
Christina never lip synchs because she can actually sing. Rihanna did lip synch in the beginning of her career but now she never does. She only uses backing tracks.
Thalía used to lip-sync every time, since 1990 she only did lip-syncing (there were times she sang live but I can count them with one hand). But since 2010 she left that behind and she sings live always.
(No offense but.... 
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