Here the author should've clearly explained why he (MC) didn't even try to pursue the boss..
Was he 100% sure, that he couldn't have catch on to a demon, who's flying and carrying another one..? (Wouldn't it be a lot slower that way)
We all have an understanding, that the MC is very strong by this worlds standards, and also very fast.. I don't buy it that he couldn't have catch them if he really tried, even though they had a head start.
I mean it pretty sure was a bother for the MC to go all that way there, killing all those mobs. Now it just looks that he intentionally let him escape, and was just dicking around doing nothing after shooting that explosion magic on him..
It just makes his motives/this scene look stupid and conflicting (and that means making the writing look bad)