I guess because they started to develope the love route, when mity starts a new book or the 2nd route it always takes up more time
correct, if I remember correctly, in one of MITYs post there was a talk about releases and how they start and finish of book 3 basically took 2 months instead of one. the start took longer due to all the new screens, backgrounds and the likes that had to be made all at once to start out. You have exposition dumps and all the new elements had to be designed - Like the underground or the city building. (the underground tunnels should've been something along the lines of Dungeon Master to begin with, meaning rotating and the likes and moving forward but got scrapped since even MITY didn't know what was going on) and the ends take long because all the events had to be tied up to an ending that is satisfying and that may or may not involve multiple scenes which needs drawing.
I suspect some elements - if not all - to be recurring, but also new to be put into place, and as he also stated, in the love route, there's gonna be a lot of characters involved, that'll have very little screentime, so there's extra work ahead in terms of drawing them.