Press on the gas but car stays at a slow constant speed no matter how hard I press the gas?

So I got a 2001 Honda civic ex manual 5 speed Bought it from a private dealer. (It has a cold air intake in it and a huge exaust) I have a air leak right where the headers meet the exaust pipe(I don't know if this helps) but when I get into 3rd gear or above the car won't go faster the harder I push the gas petal down. I can floor my petal and it will still go as fast as if I was barely pushing the petal down. It's sad cause the car goes pretty slow and it's upsetting when a old lady can pass you haha but also I noticed my rpms match my mph for example here's my speeds for each gear
1st - 1 - 20mph
2nd - 20 - 30mph
3rd - 30 - 40mph
4th - 40 - 50mph
5th - 50 and above
also any tips to gain more HP? And also help my speed with the gears so I'm not shifting so short timed?

Clogged o2 sensor

Vacuum leaks can degrade your performance.