The 6 Plus can only run iOS 12 max. The 6S Plus can run iOS 15.Try the latest iOS 15 that works on the device.
My college has it on his 6S+ and refuses to swap his phone as it is still being supported with software updates.
i'm using iOS 15 on my SE and it works great.
But check battery status, if its to degraded it will throttle and that affects performance.