I use autocorrect because it lets me type sloppily enough that I would make a
lot of spelling mistakes without it, frequently making words unintelligible. Of course, I could type reasonably accurately if I slowed down, but in theory, with autocorrect I can type faster and still end up with the right text most of the time. Yes, miscorrections happen, more often than I'd like, but not enough that avoiding them would be worth even a small speed penalty; virtual keyboards are slow enough as they are.
…On the other hand, my teenage younger sister has always used her phone with autocorrect disabled (just because she doesn't like the capitalization), and she types pretty quickly. Maybe it's just a matter of training yourself with autocorrect off, and eventually you can type accurately while keeping the same speed. But I'm not sure. There's no doubt that training would help, but can it make up the full difference, compared to a user well-trained with autocorrect on? (…And for that matter, if I care about speed so much, shouldn't I switch to a swipe-style keyboard? But that's a separate discussion.)
Anyway, the fundamental problem is that without the tactile feedback of a physical keyboard, it's much easier for your fingers to get misaligned, so the relative positions of the taps might be reasonably correct but the absolute positions are totally messed up. Even though you can see when you make a typo, humans have pretty high latency in responding to visual feedback, and you only get the feedback after the fact (whereas on a physical keyboard you can feel if you're in the wrong place before you've actually pressed down on the key). It seems more efficient to just type the whole word at once, and even if you were misaligned, most of the time autocorrect will figure out what you meant. (Especially if you keep going with the sentence, which can change previous words after they were seemingly finalized. I used to go one word at a time, stopping to correct any word that turned out wrong, but after I learned to trust autocorrect a bit more I realized that some of those corrections were unnecessary. This applies especially to the word "its", which autocorrect likes to change to "it's" but will often change back when appropriate.)
Actually, I make typos on physical keyboards too - which I'd like to think is just a side effect of being a fast typist, though I'm probably less accurate than most - but my impression is that those are more often timing rather than positioning related, and I can often tell they've happened by feel alone, letting me hit option-delete to redo the word immediately without waiting for visual feedback to go through my head...