Besides Mega Man 3, which of the NES games have really gnarly slowdown? I think they mostly had their stuff together by MM4 or MM5, but I can't say.
Also, I may have been wrong on Mega Man 8 all my life and it's bothering me right now. I really like Mega Man's loadout from what I've seen from the first four bosses. Every weapon so far has a unique, distinct utility niche that's fun to toy with and they all feel fun to use across the board. The Buster/Special Weapon buttons being equipped simultaneously works way better here than they did in the PS1 X entries that originally brought the feature (to the point where I am adamant Mega Man 11 needs to bring this back), and some of the level gimmicks are much more clever than I remember them being. Mega Man R-Type is so good it needs to have been its own spin-off years ago.
I hope Mega Man 8 nosedives soon into unplayable crap, because I'm not comfortable lumping this much praise on it.