How do I make my bike faster - Mongoose 20 in. Boys Axe Bike

You get what you pay for.
The setting for speed are 5,10,16 mph.
You want anything faster you would have to do major rewire of the bike to over volt and current on the motor and additional batteries - cheaper to buy a faster bike.

Surging is usually at the throttles and idle air control motor. Cleaning them usually takes care of this problem. I haven't worked on bikes since the 80's. Surging back then was always dirty carbs. I am assuming that the new bikes are fuel injected and have an IAC motor.

If you want to make your bike faster - pedal faster.

It worked for Chris Froome and Bradley Wiggins.

Why do you think the bike would be slower? a cdi is mearly a box that regulates ignition pulses. There is no power to be generated here, unless you're flashing your ecu and re-mapping yout ignition table. Just run the original CDI. It's easier and cheaper to make yourself faster than your bike faster.

There's an idle adjustment screw. The bike runs faster when cold because the fuel\air adjustment is richer on a cold engine. When the bike warms up, the fuel becomes more lean in mixture because the heat causes the fuel to expand, i.e., less molecules of fuel to the firing sequence of the piston. When the bike is warmed up, after say ten minutes, the heat causes a leaner mixture, so adjust the idle on a hot engine (about 1000 rpm more or less). The bike will idle faster when cold, then drop rpm's as the engine warms up. Good luck

Change out the gears with a higher ratio that turns the wheels faster for the same motor RPM. Another method is find a motor that has a higher rpm capability.

Pedal faster. Or get a bigger drive wheel. Good luck with that.

If it was the engine, chances are you'd feel it in the lower gears before 70KMh
I'd have the wheel balance checked at a bike shop.

Road bikes are faster due to their generally higher gearing and especially due to their narrower tires that pump to much higher pressures. Think of your bike like a boat. When you start a boat it gets up to speed and eventually is up on top of the water (what is called planing) and then you can actually reduce the throttle and mantain the same speed. In the same way narrow tire road bike riders can get their bikes up to speed and actually throttle back and keep the bike going at a similar tempo. The tires on a hybrid bike do not inflate to the 100-130 that many road bikes tires do. We used to have a pair of racing wheels and tires at my bike shop that we would let hybrid owners borrow. They could not believe the difference.