The honest truth... The S7 Edge lags mainly only under these conditions:
1) While apps are updating
2) While connecting to Bluetooth
3) While connecting a charger.
4) While typing.
And of course it doesn't lag all the time during those situations or else I'd be ******** constantly haha. Those are just the one times that it does lag. And generally speaking, it's been the closest to lag free of any Android phone I've ever owned.
NOW, here is the important part.
All Android phones will eventually lag after system updates. I don't care what ANYONE on this forum says. Especially when going to a new version number. Forget it. Your phone is now a ticking time bomb that will eventually become a laggy, unfixable mess no matter how many system restores you do.
Now I haven't rooted and messed with custom ROMs in a while... But back then, it was the same thing. As soon as you cross the line into new version numbers... Your phone is toast. It's just gonna be a constant hassle until the day you get a new phone. It just is. I have had:
G1, G2, Mytouch 4G Slide, HTC One X, Galaxy S3, Galaxy S4, Note 3, Note 4, Moto X Pure, S7 Edge.
I'd say at this point I can definitive say that I am right about this. Every single one of them became a laggy shadow of its former self after a major system update. Also, using SD cards will make the issues worse. Also, installing custom ROMs only makes things better because by the time issues arise, you were already about to wipe and flash another custom ROM anyway. If you had waited another week or two everything would have become a mess.
That being said, I love Android, and not like I'd ever get an iPhone... Plus, one look at an Apple forum tells you that no matter how holier than thou they all try to act, their iPhone's are just as bad, if not worse haha.