f l y

Was the fly real?

He never swatted and acted like he felt nothing there.

It looks inserted when the video is slowed down because either the fly is approaching his head at phenomenal speeds or the missing frames which would show the fly as it comes into land would be visible and they're not.
So either a particularly fast fly or a fake fly, it should also be noted that there is very little visual detail on the fly when video is zoomed in but that hair strands are very obvious.

Suspicious as to why they would insert except to distract from the poor performance of a certain other candidate.

so what you are saying it was faked! intersting

Yeah, I would like to say it is faked but I don't know how to disprove that theory based on video evidence alone. The video evidence would lead to it being faked.
Still it's very odd he didn't notice it, Hillary also had a fly land on her in 2nd debate in 2016 which also didn't make her react. Maybe it's the heat of studio lights and they can't feel it.
And then we still don't know a reason why anyone would have thought a fly on his head would work as a diversion for anything and that's enough reason to say "Just Saying it looks odd"

That last part is the crazy sounding bit because makes little or no sense why they would have done a fake fly. However, People are talking about the Fly more than the content of debate but it still doesn't validate a reason.