It takes me also more like 30 min for 100 reviews. But I do read up sentences and explanations again and also write down kanji, which I got wrong. (I just felt like looking more closely and seeing the differences helped me, and this worked best for writing it down. Especially in later levels, where some kanjis just have a radical as difference.)
Maybe I would be faster, if I really just write and move on. No idea.
But yeah I do feel very slow. XD Especially when I compared it to the time people spend on each level, they were all so much quicker than me. (I mean this thread here Drop your level up chart here 📈)
I think it would work better, if one tries to stick to the time, where one can review things again, I sometimes didn’t do every day and thus forgetting things. It probably would also be best doing reviews throughout the day. (Then likely you won’t have 100 reviews at once though, unless maybe you added all the new items right away.)