You can only improve a phone to the limits of its hardware with another rom.
Compared to modern iPhones, all budget phones will lag severely, no matter what you do.
Get comparable hardware to an iPhone, lose TouchWiz and then watch a fair comparison.
If iOS is still the one for you, great. We vote with our wallets and that's the way it ought to always be - none of these companies are our friends. Get what YOU like.
At least then however you're not comparing apples to oranges when the hardware playing field is level.
I use an M8. It's FAR from laggy. This spring even faster models are coming out.
I think that iPhones have too much design lag, starting with the one button - makes about as much sense to me as a one button mouse, along with having to go outside apps to get to settings - design lag. But - that's just my opinion and so I chose my preference.