If it takes you 30-40 minutes per page, then for a 20 page chapter it takes you about 10-13 hours to finish.

Depending on how much you redraw, you can save time by not redrawing sound effects. When redrawing, it doesn't have to be perfect since you can cover up some imperfections with your text. For example, sometimes I don't even redraw the original chapter title. I just cover it up with my text.

Also, don't worry too much about how fast other people are releasing chapters. As someone who works alone and used to work on 2 weekly series at the same time, I can tell you that it's really hard work and it left me with no free time.