No, touchwiz is NOT an operating system. All the background processes running an a samsung is not "touchwiz" they are just apps, not the os itself. The bloat apps is what is eating up processes, not the touchwiz launcher. You can essentially root it and get rid of all those apps while still keeping the touch wiz launcher. Do I need to point you to a wiki article which states the difference? Here, It says in the first paragraph:
http://en.wikipedia.org/wiki/TouchWiz
Now mind you, I hate the preloaded apps that samsung puts on the phone. Way way too many. If I wanted all that crap I can install it from the play store. That's why I got a nexus 5. I still use the s3 from time to time and love it, but I use the nexus 5 99% of the time.
Anyway, app opening speed isn't all that important (although I personally think 3 seconds is slow if you're used to apps opening almost instantly or within only 1 second), but if you notice, the apps that do load faster that require an INTERNET CONNECTION open faster on the note 4, not because of the higher ram, but because of the faster downloading speed. A split second of app opening isn't a big deal, but being able to fully load a web page or web data super fast is pretty important. In THAT regard, the iphone 6+ lags pretty far behind in comparison. When I say "lag" I don't mean it in a negative way, just a little slower.
What this shows is that even with all the bloat crap running in the background, the note 4 STILL runs circles around the iphone 6+...and THAT is a huge accomplishment! lol. And again, put lollipop on it so it cam support ARM and it will be even faster.