I'm not sure I'd call it "fast" either. For the first three months babies can barely see anything, and for at least nine they can't form anything even remotely resembling speech and can't walk. What's amazing is that all this learning is very sparsely supervised and all "subsystems" train at the same time.
This is a human peculiarity. Many animal babies are born with fully developed abilities. I am sure we have all seen the NatGeo videos of antelope babies stumbling 2-3 times and then immediately start walking and even running.
Human babies have bigger head-to-body ratio compared to all other species due to our brain being bigger. Our babies have to be born earlier or otherwise they cannot make it out of the mother alive.
Outside of that, we develop pretty quickly. As you pointed out, everything in us is developing in parallel which is quite impressive.
