I have a manual transmission (2008 Honda Fit Sport) and recently got my car serviced, including oil change and "fuel system cleaning", at a shop I have never used before. After the service, my car seems to have significantly worse acceleration. Specifically, if I press the accelerator down aggressively, the engine makes a lot more noise than it used to and doesn't pull nearly as strongly as it used to, especially in the middle gears at low(ish) RPM. The noise is a buzzing sound, and dies down if I let up on the accelerator. When trying to accelerate, the revs seem to climb slowly. Opening the throttle wider makes the sound louder, but doesn't make the revs or road speed go up any faster.
If I drive very gently, it's hard to tell whether the noise or acceleration is worse than it used to be. I can still creep up to highway speed at a leisurely pace, but I can't get there urgently.
What's wrong with my car? What did the service guys do to it? It was definitely not like this before they got their hands on it.