It doesn't. I've been an Android user since the Nexus One. Android ran amazing on that phone, on pretty much every Nexus phone, and on my Pixel. It ran like crap on the LG G2x, a phone so flawed they lost a class action lawsuit about it. It ran mediocre on my Galaxy S2 and S4, but much better once I used Cyanogenmod. It ran beautifully on my Moto X Gen 1.
Android, just like iOS or any other OS, requires non-crap hardware (not amazing, just well designed) and an OEM that doesn't bloat it up with tons of crap. Newer versions of iOS run like crap on older iPhones, because the OS is designed for newer hardware. Android runs beautifully on well designed hardware, even inexpensive hardware like the Moto G. It runs great without bloated skins and shovelware that "adds value".
Being a long-time iPhone user and recently using Android for work (high-spec Nexus models) I can safely say that Android has a _long_ way to go both in terms of responsiveness and design.
I hear this from folks, and I must be missing something. My wife has an iphone as do many of my users/clients and so I use it a lot, and own an iPad myself. I really don't see a responsiveness difference. Design is more subjective.
