XP always ran appreciably better on budget machines if the Classic theme was selected.
Vista was slow as molasses, and the slick look didn't help one bit. It didn't help that the Vista era saw a major shift to home users buying laptops with less than good integrated video either. A visit to 'Performance....' in Control Panel always showed that performance could be increased by knocking out some or all of the slick features.
Windows 7 was given the Aero theme because speed was clawed back from other areas, but still the 'Perfromance.....' utility in Control panel advised removal of some or all of Aero if the user wanted to get more overall performance.
So, now Microsoft have an OS which looks the same across the various platforms and will perform well across the various platforms, assuming that the screen resolution is greater than 1024 x 600. It makes economic sense for Microsoft and the end user, neither having to pay top dollar to get all that the OS promises.
The Windows 8 interface is flat, but I would not call it 'distracting' exactly, especially in Windows 8 app mode where there are NO borders.. Well, is branding more important than user satisfaction if your branding makes me hate your brand? I don't care if it's flat or shiny. I want things arranged and functioning on my desktop like they have always done without a problem. Microsoft has created a problem and doesn't want to fix it because some marketing **** slobbered about cross-platform branding during a meeting and some fool bought it.