How do you keep a Burger King maintenance job?

I got a job at a Burger King near my house. I've been working there for three weeks and I'm 14. It's a burger king/pizza hut/kfc/motel, and I am a maintenance worker, so I wash trays, take out trash, mop floors, clean washrooms etc. I do everything related to cleaning. But I work the last shift of the day, so I have to clean up everything so it's clean in the morning. Usually there is a big dinner rush from 6:00 to 8:30 and after the rush I start to do everything for the last time. It usually takes more than an hour and a half to do this, and my shifts end at 9:00 and 10:00, which gives me not enough time to finish everything before my shift ends. We use a punch out system, which calculates our paychecks, and if I stay longer than I'm supposed to, I will get more money. I am pretty much getting paid extra for my slowness. Will I get in trouble? Will the managers see that I'm slow and fire me? I talked to a manager about this, and he said it's ok because I'm getting all my work done, but I don't know if he is just trying to sound nice or anything. Also, can someone provide me with tips on how to be a maintenance worker please? Thank you