Exhaust replacement & engine tuning

Yesterday I changed my exhaust (catalytic converter & mid-muffler). The feeling of driving with new exhaust on is very different. But there is one problem. I experience some hesitation with the pick-up while in gear 1 and gear 2. The car start accelerating slowly (like 0-100km in 20 seconds) all of sudden the car accelerate faster (like 0-100km in 15 seconds).

This problem occur before I change the exhaust, but after the replacement, it happen quite often.

Can mercedes experts like you help me with this problem? The mechanic recommended me to go for engine tuning. May I know what are the things in the engine involved in engine tuning?

Thank you in advance.
Rizami