dabac

And the sad little thing they post as "specification" isn't much help either. Whyh not try counting the teeth, and we'll know what we are up against?
Meantime let's guess at 22-32-42 front and a 14-28 on the rear.

Wide? knobbly?

Smooth, narrow and pumped harder can help.

Nothing wrong with that, but a cheap MTB is by definition a heavy MTB. Makes a difference when you've got to get it moving.

Doesn't really say a lot. Bikes can be built farther or closer to the ground, and the key measurement is how straight/bent your leg is when you're in the saddle and the pedal at the bottom.

Good start.

From a functionality perspective those are usually perfectly fine.

Again, you have to differ between bike speed over ground, and what the rider is doing.

There's a word for it -cadence - that's how fast the rider is turning the pedals. These days the stock recommendation is that you should aim between 80-100 turns/ minute. If you can't crank at that pace, you're probably using a too heavy gear, which increase the risk of knee troubles.

So first you find out "how hard can I crank and still keep up the 80-100 turns?" Then you see how much speed that effort will buy you on a certain bike.

Hitting a cadence of 100 is well within reach for most of us, maybe with a little training. Even if you can only make 90, that should still have you going 20+ MPH

That's called spinning out, but unless you have something very special going on with your body, that shouldn't happen until well past 20MPH

Have a look at Sheldon's gear calculator Enter your tooth counts, set it to display MPH at cadence and see what you get.

Most MTB-based bikes would have you spinning out at 30 MPH, you need serious road gearing to keep inputting power at that speed. But outside a race setting, who needs it? I'm quite happy to coast those few seconds out instead.