Anyone who thinks that you can cut out half of Monster with no problem didn't really "get" it.
I'm not saying "lol 2 deep 4 u", but there was a lot of significance in many of those "filler" stories, and where there wasn't direct relation to the plot, there were simply some beautiful, awe-inspiring scenes. You probably took a lot more from those "slow, boring parts" than you realize.
That said, I cannot fathom how this will work and still retain its flavor. IIRC, the word is that the production company greenlighted the adaption for two movies, so we have 4-6 hours feasibly. The last I heard from Josh Olson, who is doing the screenplay adaption, is something along the lines of: "It's kind of sad when I go onto forums where people are discussing the series. I'm reading up about everyone's favorite parts and many of those are the ones that I had just cut from my script."