@hami_cheeta!
- Its possible that the mechanic has leaned the mixture of your bike.
- Air filter has engine oil on it (the oil helps clean the air).
Brand new bike has a rich mixture, to ensure smooth running process of the engine. After first 3-5k kilometers of running, mechanics usually make the mixture lean. Its possible that the mechanic who tuned your bike has reduce the fuel flow (leaned the mixture), which is why you are feeling a slight decrease in its power.
As for the oil on the air filter ... ride around a few kilometers. After approx 50-60kms, the oil on the filter will start to dry, and you will start to feel that power is coming back.