Why is denji wanting sex a character flaw? What exactly is the "development" that you are talking about?
30 chapters of a single fight? At least fact check your claims before saying some bullshit, this fight started in chapter 215, 14 chapters of a fight, each chapter is 15 pages, and some of these chapters have little to no dialogue, now if you compare this to the lenght from denji vs reze in the movie its probably going to be about the same if not less due to the dialogue. It just sounds like your want fights to last 3 pages or something.
I read more weekly manga then you've read manga, just because something new isn't happening every chapter doesn't mean its dragging, the chapters are 15 pages long, this "dragging" that so many are complaining would take you a couple of minutes to read from beginning to end, most of it is a fight. When this gets an adaptation, all of this will be in 1 episode.