Most people's complaints include tabs reloading after a very short amount of time, resprings and crashes. Probably due to the Plus's 1GB of RAM and the GPU being required to render things @3x and downscaling it to 1080p.
The 6S will probably be much better if it gains 2GB RAM and CPU/GPU upgrades.
Hi,
I did feel the same about safari on my iPhone 6 Plus but after turning to Google Chrome I'm much happier with much less reloads. Theirs lots of different options. I also use Atomic Browser although support has stopped for it I get no tabs reloads so this leads me to believe it's not just internal RAM but how Safari performs.