You'll see people chime in here saying: "I see no lag". These people are either lying or simply don't notice the lag.
This is because lag is objective, not subjective. Speed can be measured and it's somewhat easy for people to notice when a computer is slow or slower than what they're used to.
I started a poll about iPhone 6 Plus Lag literally days after the new iPhones launched (linked to below). I was hoping that iOS 8.1 would fix it and it doesn't. The lag is as present as ever. One major pattern I notice with people complaining about lag is transition effects. For instance, switching between Apps. Or opening an App (e.g., calculator). There's a longer delay I find for things like drawing up the UI of an App, particularly when it hasn't been opened. Try opening the calculator App on an iPhone 5S and then on the 6 Plus to see an example of lag drawing up the UI.
Also, pressing the home bottom to be taken home often results in a longer delay before anything happens than my 5S. There's lots of other examples.
The iPhone 6 Plus feels exactly as the iPad 3 did. It's annoying as hell but I've decided to stay with the 6 Plus. I'm at a point where I don't think the iPhone is that much better than an Android smartphone like a Note 4 or Sony Z3. The lag is part of this reason.
I also agree with you about the slippery construction and awkwardness. The top and bottom bezels are bloody huge. They need to make this thing less tall physically. That would be a start in helping with the size.
https://forums.macrumors.com/threads/1791582/
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Dude the reduce transparency won't really change anything. I've had that setting in place since day one and it's lagville.
You're experiencing the placebo effect.