It's the RAM. 6+ has 1 GB, 6S+ has 2GB.
It's laggy ! Always laggy . Can't imagine this phone running iOS 11 or higher iOS versions.I Understand the 6+ only has 1 GB of RAM. What I was indicating, is that the iPhone 6 Plus is already outdated in terms of hardware and isn't manufactured anymore, however its still fully functional and widely used with current iOS support.