You cannot call Race 3 a performance, Salman wasn't even trying there. It was very disheartening to see a veteran actor care so less about his performance.
The worst Khan performance this decade has to be Tubelight.
Aamir was also very bad in Dhoom 3 in both roles.
Worst SRK performance would be tough to say because I don't think he has a truly bad performance this decade. Yes, his acting in the beginning of CE was pretty bad, and he was bad as Shekhar in Ra. One, but he was good in latter half of CE and as G. One. Zero wasn't a bad performance, though it initially looked that way because of the character and movie. I guess I'll go with JHMS just because he was unable to bring any gravitas to the character.
---------- This is my opinion, please keep that in mind before replying ----------