Awkward timing on the time skip. It feels like they tried wrapping up everything in one fell swoop without actually putting thought into the process. It makes the manga feel cheap.
This was such a boring introduction to the demons. Also, having demons that are that powerful kind of throws off the power balance in the story. How did humans manage even to live this long?
Hopefully, by the end of this arc, the MC will kill the summoners. If not I'd be disappointed. But this is a low-grade C-tier manga. So I doubt he will.