iPhone 6s Plus lags when typing and flashes between screens plus pulls up apps and pages when not selected

Just purchased another iPhone 6S after mine broke. This is my second iPhone 6S in a month as I sent the other back due to running very slow. I have all background refresh turned off on my apps, I have all locations turned off except when using an app. I close out of all screens after I’m done with them and my phone still lags. I type and it doesn’t register I typed for seconds after scrambling all the letters and not even making the words correct. If I touch something on the screen it takes seconds to respond and sometimes it selects something else I didn’t even touch. If I pull up my camera or pull up Snapchat the camera sometimes takes up to a minute to come up or doesn’t come up at all, just a black screen. Sometimes when I pull up snap and the camera comes up then the phone goes all crazy and flips to another app. My first iPhone 6 which I had since it came out worked fine however I had to purchase another due to the cracked screen.

Is the phone running significantly slow and lagging because the age of the phone vs the updates or is it lagging because something is wrong with the motherboard inside?

Im not ready to pay thousands on a new phone when the 6 does just what I need but I feel that something is wrong with it and want it corrected.

​[Re-Titled by Moderator]