1999 Chevrolet Cavalier not sure

My cavalier is very slow I bought this car from someone so I don't know the history of this car but t has not power I'm not sure what the problem is but I was looking at my engine and I found that this connector that when pulled makes the car rev but even with that disconnected I still car rev the car with the gas pedal I keep putting it back in place but every time I put it on drive I guess it slips off I never feel it. My question is is that why my car had no power? Also my car cannot make it up hills at all one day I pushed it really hard and it reved really loud andwhen I looked back there was smoke I'm not sure where it came from it still drives fine just slow if that conneter isn't the problem what could be? Please help this is first car I just want to fix it. Thank you.