I don't really think it was going to fast , people fall for others super quick sometimes. Look at all she's done for him in the first 6 chapters and add to that never having a friend before plus finding her cute. It would be weird if he didn't like her already , the only reason though why he knows instead of thinking it was just friendship for however long is because of the other girl.
I do think the other person has a good point, though you're absolutely correct that him falling hard and fast (even if it took a third party to articulate it) is not out of the ordinary.
I think it comes down to Shimohoshizaki-san just...not having friends before. Even Ujioka wasn't sure if what he felt was "my first friend!" or something more, it just happened to be "something more" for him.
Could be the case for her, even - but with no frame of reference, "best friends" might seem like the answer, when she's always been alone before that point.
I think "time" is correct here. I also think it should
not devolve into dumb misunderstanding-fueled shenanigans and love triangles and them not saying their thoughts out loud with one another at every opportunity.
Which, I kinda suspect won't happen. But a slower-paced burn of him falling harder and her learning to fall in tandem--will continuing to have fun and learn more about one another--would make for a very rewarding and cute series.