It's a HiDPI problem, which is a software problem. Windows running in Boot Camp on the very same hardware doesn not exhibit the same slow scrolling behavior. Therefore, not a hardware problem.
A software update has been shown to alleviate the slow scrolling. Software updates don't change the hardware.
Therefore, not a hardware problem.
There is absolutely nothing here to indicate that the hardware is too slow. Even systems many years ago, with much weaker GPU's, had no issues driving the same number of pixels (on the 30" Cinema Display).
The problem ONLY occurs when HiDPI mode is used, and based on the fact that it's fixed by SOFTWARE optimization, it points to an issue with the HiDPI implementation in SOFTWARE.
It's not a hardware problem. There's nothing at all to indicate it's a hardware problem. All the data points to a software issue, which has now been resolved via a software update, which doesn't change the hardware. The problem goes away on the same hardware, when running updated software. Therefore it cannot be a hardware problem.
How many different ways can I say this? It's not a hardware problem.