You shouldn't be typing at a constant / consistent speed. When you're at a section that's easy to type, speed up. When you're at a section that's a bit more tricky, slow down.

Different words fall under our fingers differently; sometimes the process of moving from one key to the next is very smooth, sometimes your fingers get in the way of each other.

As you type more you'll get a better feel for what is and isn't easy to type, and you'll gradually adjust your fingerings to make some difficult sections easier, letting you type at closer to max-speed for longer amounts of time, and thus increasing your wpm.

Muscle memory is lowkey nice. Just practice on other websites too, or maybe record your progress to see how far you go to get more motivated.