You need to learn some common sense. Just because something new is offered, you don't have to jump on it.
Or you are you one of those types who had the iphone 4, then the 4s, then the 5, then the 5s, and now the 6? "I'lll have to switch to a new phone someday, so I'd better switch now to whatever the current one is!!!"
In my experience, the fastest OS for a device is the original it came with. Leopard on my MacBook Pro is very fast. Yosemite is slow (especially on only 2GB RAM)
Each update seems to make it slower and slower. If this is as true as it seems, then the best thing to do is to use the oldest supported release.