As someone who reads Japanese, reading all kana can be slow because you are reading one syllable at a time. It makes going back to old video games and reading children’s media tedious.

Once you know enough kanji though, you can read incredibly fast. Depending on the material, I can speedread faster in Japanese than I can in English.

This is because kanji is meant to be recognized at a glance rather than read in your head. The kana in Japanese sentence is supposed to provide grammatical context. So in stead of reading “Inu ga ie ni nemashita” in my head, I’m seeing “Dog, in house, slept” from a glance.

So the downside to this system is that you’re spending most of your education learning every character you’ll need, but the upside is it can make reading very efficient once you have got it down. I think it’s part of the reason Japan still has a pretty robust book culture.