Phone issue or is IOS 10 the problem?

I finally gave in and had my touch disease ip6+ replaced at the Apple Store that's been out of commission for almost a year. Now that I've fixed it, I've realized that it's extremely laggy when opening and closing apps. It's almost to the point where I don't even want to use it and just trade it in.

None of my iPads have this issue, but they do have 2gb of RAM.... Is the lack of RAM the reason IOS 10 is such a bad experience on this phone or could the phone be defective?