Only the last 20 minutes of this movie matters

I guess that's true if you're the sort who watch horror movies just for the kills... in which case those compilations on Youtube that edit horror films down to just the gore moments oughtta suit you fine... no need to ever 'waste' time watching an entire movie.
For me, the first part of this movie is important for setting up the characters... taking the time to let us get to know them and care about them... despite the fact that they are VERY annoying. The first part is what makes this movie special because a Hollywood movie, particularly a horror movie, would NEVER let that much time be spent on setting up characters... they think the audience is just a bunch of bloodthirsty goons so you get 2D characters who are just there to squirt blood every 15 minutes or so.
If it wasn't for the first part of this movie there'd be no reason to watch it at all.

Agree.
(Spoiler alert, please don't read further.)
Sure, the film needs some plots to get to the last 20 minutes. But, did it have to be so long, so pointless, so boring ? Basically a long 70 minutes of shaking camera shot, not much going on. Those obnoxious young adults were killed in the end, it was wonderful.