Experiencing same problems concerning using the spot tool in Lightroom CC (2015) as with LR 5.
Using iMac 5K
AMD Radeon R9 M290X 2048 MB
Wacom Intuos 3. Performance with tablet slower than mouse but still lots of lag with the mouse. Again, slower when 1:1 but also plenty of lag when FIT.
Anyone having similar experience with LR CC - thoughts much appreciated?
Try unchecking "Use Graphics Processor" in the Preferences/Performance pane.
The spot tool was not one of the features enhanced by GPU, however I have noticed it does actually impact its use. IE. It slows it down until I uncheck it.
More info here: GPU notes for Lightroom CC (2015)
So much better with "use Graphics Processor" unchecked.
Still not perfect but perhaps it is the best it will be?