I do have a 15-inch retina MacBook Pro (early 2013 model) and Yosemite is not as fast as I expected it should be. I also use Windows 8.1 on BootCamp and it is way faster than Yosemite. I tried putting Windows 10 and the wi-fi didn't work due probably to drivers issues, so I went back to 8.1. And, yes, 8.1 is still faster than Yosemite.
People may say Windows gets slower with time. Perhaps it was the truth a few years ago, but I am not so sure anymore. My current Windows installation is a year old, and it is still fast and zippy. My Yosemite installation, however, is sluggish.
Two things to consider, though:
- Apps really load faster under Windows 8.1. This is perhaps most apps are written for Windows and then translated to Mac (such as Google Chrome, for instance), so lots of unecessary code slowing down the apps. In addition, some Mac developers may have less funds available to make better Mac apps, since the potential revenue is lower than the one Windows developers expect (due to the fact that there are far more Windows users than Mac users and, therefore, the potential market is way bigger). It is just speculation, though. I don't know the real reason, but apps are loading faster on Windows.
- OS X drags a lot of graphical resources. When I run OS X on my 15-inch retina display at a resolution that resembles 1920x1200 (which means, OS X renders images at 3840x2400), then it is slow. When I run it at 1440x900 (rendering at 2880x1800), then it is much better. The thing is, I like running it at 1920x1200, so I can have more real estate to work on. When I connect my 27-inch external monitor (at 2560x1440), then it is faster (as it renders images at 2560x1440) than running on my retina display. The bottom line is: the way OS X renders images for retina displays may be very nice for the purposes of keeping the aspect ratio, but it demands a lot from the hardware. Windows, on the other hand, does not have these issues.
I installed Windows 10 on my Surface 3 (the non-Pro version, with the Atom processor). I am impressed. Windows 8.1 was OK but lagged a bit. Windows 10 flies. Microsoft did a great job. But Yosemite, well, all these graphic resources were overkill. I am disappointed at Apple in this respect.