I bought one of the new Mac mini's. It's hooked up only to a single non-HiDPI monitor. And for the first time in five years (the five years since I've had my rMBP), the whole system feels amazingly smooth and responsive in comparison. There is no delay on text or window resizing.
I've always suspected, ever since I got that rMBP, that the GFX chipset in it just wasn't up to the task of running that many pixels. After five years of annoying lag all over the system, this Mac feels so refreshing to use.