“I never expected them to take this long”
They fuck do you mean? YOU WROTE IT!!! I’ve seen authors do this so many times and it enrages me every time. I love slow burn and I don’t even think the pace in this manga is bad, nor do I think the pace should go any faster. So why are slow burn authors always going “wow this is taking a while” when THEY CONTROL THE PACE???!!!
Often when you're first setting out to write a story, you have an idea in your head of where it's going to go and how long it might take to get there. But then once you start writing, you start fleshing out the details and fill in various gaps. You get sidetracked in various ways, building out parts of the story you hadn't even considered at the time, and eventually you realize over the course of writing that you grossly misjudged the scale of the effort. Stories can be living things, in a manner of speaking.
This means it can actually be very difficult to accurately gauge the time it's gonna take when you're just staring out, so I'd say the statement is actually quite apt.