Re: Best launcher for Marshmallow
Most people here are going to tell you Google Now or Nova, as the general consensus is that those are the two best launchers around. I agree with that consensus as well. Google Now is simple, but effective. Nova provides lots of customization without drastically changing the UI as some launchers do. I've used both, but I've mostly used Nova for the last year.
IDK that you're going to see dramatically different performance for a launcher just because you upgraded from Lollipop to Marshmallow. It's not so much a matter of finding the "Best launcher for Marshmallow" as it is just a matter of finding the launcher that meets your needs best, regardless of Android version.