Apple slows down older iPhones!!
Apple makes older phones slower with a software update.
The iPhone 6 is also seen as an old phone and becomes considerably slower.
Apple says they do this because the older batteries last less after charging and try to compensate by making them slower.
but many people disagree because not every iPhone battery becomes poorer, so they think it's a trick to let people buy a new iPhone.
There is a way to replace your battery but it is not clear if this will prevent it from slowing down and it will avoid you warranty.
https://nl.ifixit.com/Guide/iPhone+6+Battery+Replacement/29363?lang=en