As someone who did all the fast levels at full speed and raced to 60 in only 353 days, I definitely don’t recommend speedrunning. Or rather, I’d say speedrun the first half and take your time after that. The last few levels have mostly obscure kanji and words that don’t matter much and it’s a huge slog to get through. I was spending over three hours a day on reviews at the peak, and I barely remembered them anyway.
Adding on the fast levels : I did them and you should prepare yourself to have +500 daily reviews. The other levels, I needed 6-8 days. It is very grueling.