I dont think it's out of character for mike to be so late responding to Will's coming out. You guys, this is Mike that we are talking about. That boy just had the biggest revelation in his entire life. He always needs time to work trough things like this. The rotors in his head are turning but very slowly. To me it almost looks like he snaps back to reality before he says something. He is putting ao many things together at this point. Imagine how many moments are rushing trough his head right now. And thats under the assumption that he is indeed straight, imagine what is happening in that mind of his if he actually likes Will back (yes i still have a flicker of hope)
Dont get me wrong, theres definitely a bigger conversation to be had. And its essential for them to have this conversation. If they dont have it then mike is indeed a bad friend. I would beg the duffers to bring this to a good ending if they hadnt finished filming yet.
This is the first time ive been let down by the duffers with the release of stranger things eps. I believe they can right it with the finale