It means you did it very well , it has the same meaning of you nailed it . Those 3 expressions mean you're very good at doing something

They are complements that says you are doing a great job. It really depends on the context if the literal meaning is applied or not.

In American English, it is an idiom for "you are doing an awesome job". It is better than good.

It can also mean the opposite! It depends on the context! You are killing it! Beethoven would have turned in his grave upon hearing your playing!

"You're killing it!" Or "you're slaying" Are both compliments. "You're killing it" can mean "you're doing a good job" example: John: "i think i want to quit karate." Adam: "why? You're killing it! You're already a blackbelt." "You're slaying" is usually used to talk about someones appearance. It can mean "you look stunning" Example: Rachel: "does my outfit look good?" Sarah: "yeah! You're slaying!"