The whole lag issue is ridiculous... I am currently using a three year old Nexus 5 that has minimum lag issues. Sure TouchWiz is gonna be worse than Stock Android for lag but I have seen high end Windows gaming PCs experience serious lag, freezes, and reboots. Why? It depends a lot on how the user maintains and configures the system but even worse is the crap programs people like to install. All modern operating systems run well but are NOT perfect.
The comments that iOS is better is BS. My GF just bought a new iPhone 6s and I have seen lag using it day one and she has been complaining of late sometimes the messenger app freezes on start. LOL