I have a similar setup 2.6Ghz 16GB late 2012 Macmini and do not suffer the same issues. Mine runs quick and smooth.
However, some folks over on Luminous Landscape have been running into performaces issues with large files (Canon 5Ds) and high-res monitors. There are some ideas there you might want to check out. http://forum.luminous-landscape.com/index.php?topic=111999.0
Do you have a 4K or 5K monitor? How large in megapixels are the original photos? (Do not tell me megabytes)
Are you doing a lot of brushing and/or spot healing on individual photos?
I'm using a 2.5K desktop (1440p). Thanks for the link Tim, I've tried a couple of the options mentioned and resizing the Lr window so it's significantly smaller has helped. However, kind of negates having a 27" screen!
@dj_paige - A fair bit of brushing not so much on the spot healing side of things. We're talking 4 or 5 brushes with a fair bit of coverage. I'm working with 24 Megapixels photos.
It seems as though some kind of optimization needs to be put in place for higher res displays. If I'm experiencing this on a 2.5k display, I can't imagine what people on 5K displays are experiencing.
During brushing, do you have the Auto Mask turned on? Auto Mask is handy when brushing next to an edge, but it does slow the brushing process considerably. Turn it off whenever it is not needed for the editing you are doing.
Thanks for the suggestion Joe. I have been brushing with auto-mask enabled. Will give this a try.