This kind of mentality is so common with American audiences, I was surprised the originator of this conversation was German. For anyone wanting to listen to the advice of people who obviously couldn't care less about watching a movie unless something is exploding or someone is getting butchered - in other words, for the fairly mindless masses, as opposed to people wanting something more from a film (i.e. character, story, brains, emotional content) - then, by all means, skip this film. So many "big" American films have been dumbed down for similarly-minded audiences; if you enjoy that, cool, but some of us want something more from films we watch. Some of us like our brain or emotions engaged, instead of mindlessly staring up at the screen, in awe of the visuals.
I had some problems with the first 30 minutes as well but not because it was slow, it was more the romcom vibes that I thought were not all that well executed. But I am glad we got to meet the characters, at least a bit, until things begin to go very wrong. It helps one relate to them and root for their survival, turns a disaster movie into a human tragedy which is arguably what Korean cinema does best.