Car rev is slow

I just drove my car, and the rev meter is slower than before (i think). I shifted it to neutral on my driveway, and revved it, it seems to be slower than beore. The car also doesnt seem to pick up (acclerate).

Recent things done to the car
Last month - fuel injector cleaner (not machine, just a penzoil from Canadian Tire)
This month - replaced the muffler.... <-- don't think its that... cause it was fine when i first got the car back
 
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