Assuming all other things are equal, and you don't mind losing any preferences you may have changed, try starting
Lightroom by holding down Shift+Alt on the keyboard first. That way Lightroom starts with all its default settings/preferences. As an aside, I too have a reasonably powerful PC and I have no problems with
Lightroom at all, so something is definitely not right with your setup.
I think it is the Xeon CPU's. But you might try turning off the User Graphics processor in the performance tab of the LR preferences and see if that helps.
Please someone tell me what I'm doing wrong or, Adobe, please acknowledge some of your product just suck under windows environnement.
For every person complaining that Lightroom is much slower on their windows machine you can find a person complaining that Lightroom is much slower on their Mac than on their PC. This is really hard to figure out. As said, sometimes having the graphics acceleration turned on actually slows down Lightroom but in the last versions of Lightroom this has really improved to the point I now have it turned on on my machines and it certainly feels faster than having it turned off. I have an older retina MBP just like you and it's quite amazing how well it holds up even against many tricked out more modern machines. That said that monster machine should be much faster in actual operation. It could be that running Lightroom of the 12-core CPU might actually be faster than using the GPU. Whether it is windows or Mac OS should really make very little difference. If you have persistent slowness, there are a few things to try to test. One is to reset your Lightroom preferences. People have reported slowdowns on machines that have been updated many times to newer Lightroom versions where sometimes some strange corruption of the preferences files can lead to very slow Lightroom behavior. This is done by restarting Lightroom and holding the shift and alt keys. You should get a dialog such as this one: http://lightroomkillertips.com/something-acting-weird-try-resetting-lightroom-ccs-preferen ces/
Other causes of slow Lightroom behavior specific to your case can be an overactive antivirus software that rescans your catalog file every time you do an edit, having face recognition running always on Lightroom, building 1:1 previews in the background, automatic .xmp writing, and several other causes. It might make sense to investigate some of these.
Thanks everyone for you answers.
I did try to hold ALT+SHIFT while starting Lightroom, ans it seems to have worked!
Restarting LR using ALT+SHIFT really really worked. Had the same win/mac problem. Wish I knew this a few months ago!