The A8 is only 100MHz (<10%) faster than the A7, and is based on the same cyclone architecture.
It's hard to double the processing power of the A7 chip,it's already extremely fast for a mobile device and can still hold its own in real life mobile situations today.
I agree, in some ways my 6 plus is only a notch or two better than last year but it's to be expected now. You're going to be disappointed with pretty much every phone release from here on out if your expectations are double everything.