I have a iPhone 5s with iOS 8 and I don't notice any lag at all. Granted, I haven't used an Android flagship in a while so I don't have any real world comparisons. However, I have watched a bunch of reviews of Android since getting my iPhone last year and Android looks great. I will agree with you that the settings is a mess and slightly agree that TouchID doesn't work all the time but only when the home button is oily/dirty.
Don't know what to say. I've experienced lag on my iPhone 6 running the latest iOS 8 build. I've also experienced freezes, twice (once with DropBox and once with Safari). I've also experienced lack of registered touches where I'll tap something and it won't actually open or do anything until I tap it again.
It happens rarely, mind you, but it exists. I think it's well known that iOS isn't lag-proof or freeze-proof, but more that iOS hides it very well with slower animations and whatnot.
But even beyond that, as I was saying before, it's not so much that it lags or freezes or whatever (which can happen on Android as well), it's more that the overall OS feels slow to use.
Still, these early reviews of the Nexus 6 have left me wanting more in the battery section. It has a huge battery and it's still all over the place. Other than that, the Nexus 6 looks great and it's something I would consider. I would just have to get used to/deal with the large size.
That 6-inch quad HD display must be why it's showing weaker battery performance compared to the "smaller" Note 4.