Smoothness of UI is as much about software as it's about hardware. My Macbook Pro is a super-powered work horse compared to iPhone, but still many applications and web pages do not give a great smooth, experience.
Reason? Because asynchronous, multithreaded programming, which is required for smoothness, ain't trivial with abstractions that current programming languages and platform libraries provide.
With any hardware made in the past couple decades, it's entirely about software. Plus, asynchronous, multithreaded programming isn't required for a perfectly smooth interfaces.
Consider the enormous amount of instantly responsive software on 1 to 4mhz 6502s (Atari 2600, NES, C64, Apple II, Atari 8-bits, etc).. Every current platform, even the phones, can emulate those at multiple times real-time speed, so the hardware is certainly capable of it.
In the 2600s case, it didn't even have video memory and a video chip to help the CPU. The programs had to be timed to output straight to the TV at the exact speed and time the TV needed data. No threads or asynchronous events to help there.
Reason? Because asynchronous, multithreaded programming, which is required for smoothness, ain't trivial with abstractions that current programming languages and platform libraries provide.