You aren't stupid, just new. Of course you expected 30 mph, and in 26 inch wheel and regular tires with 60 psi in them, you would just get that when the battery was very full. They just don't tell you in the ad that a fat soft tire and smaller diameter will cost you speed.
Just curious, what speed are you getting to? surely still close to 25 mph right out the gate, and 20 later on?
It could be also related to your battery. If your 48v, which I assume is charged to 54.6v, sags 8v under load, then you are running in effect, 40v. A stout battery that sags only 2v under load will go much faster.
But your smaller wheel is costing you a bit of top speed too, along with tire drag if you are running 10 psi or whatever. In general, you'd expect 25-27 mph with a hard skinny tire. But a soft fat one, another few mph gone.
I would think that 2000 watts will be enough power, but you might still have to bump up to 18 cells in series battery, (64v) or even 20s 72v to get the top speed you want, and not take forever to build up to that speed.
Good advice above to spend a bit on a higher volts controller, and enough amps to reach 2000w peak. Look for one with a broader range of voltages, like 48-72, and 30 to 40 amps.
Chances are though, your current battery can't even begin to handle more than 20 amps.
so you need to ride slow awhile, till you are ready to ante up for the faster bike. Think another $500, or more.