So I just got my g37 back from having its engine replaced. Mechanic switched out clutch just to be safe as well. After driving back home i noticed that in 2nd and 3rd gear the throttle was really slow. So i went on a test ride to see what was wrong with it. I downshift into 3rd and give about 75% throttle and i only get like 25% of a response. It takes forever for my rpms to pick up. Ill be in 2nd gear but it takes me around 8 seconds to go from 2k rpms to 5k rpms. There would be some times where i could smell my clutch burning up just on a regular drive, my whole car will fill up with the smell but then it goes away. I also would keep getting a code (P2A00) even after getting the o2 replaced it still popped up. The error code has popped up three times and all those times I've taken it to the mechanic. Best way i can describe it is its kind of like a turbo lag, even though my car doesn't have a turbo its super slow at low rpms but when it starts going past 6krpms it'll start to pick up.

 

This sounds like two different issues.

1 for the clutch smell - Clutch needs to be adjusted, usually folks miss this when changing to a new clutch, the break in period should have ended and if you're still experiencing the smell it means slippage from a oil leak making the clutch slip of adjustment.
2 You probably have an exhaust leak which is why you're getting this code - A leak near the o2 sensors will lean/richen the mixture and make the car run like crap.

Again being that the engine was swap, wiring could have been pinched, exhaust could have cracked, missed a gasket, etc.