I have LR5 running on an upgraded spec iMac (late 2014) 4GHz, 32GB RAM, 5K Retina
I believe I have optimised where I can.
Typically, I'm working on 36MP images.
When I'm using, say, the 'clarity' adjustment slider, the image 'blinks' briefly a little darker as soon as I click on the slider, then 'comes back' to the setting at which I pause or finally let go of the mouse button.
In other words, moving the slider does not show real-time smooth changes. It's as if there's a noticeable time delay of about a second. This is very annoying particularly when trying to make small adjustments.
With smaller files, the time delay is less, but even so, I would not expect anything other than absolutely smooth with such a spec as I have with this iMac. In fact, I don't recall this being an issue with my previous late 2009 standard spec iMac.