First suggestion, go to your Lightroom preferences and turn off GPU acceleration.
Dunno - is it because El Capitan is proving to be a car crash of an OS?
In any event, Lightroom runs extremely well for many of us: your problem is far more likely to be with your OS or your machine than with Lightroom.
It is weird I agree. I too have a 2014 iMac 5K Retina 27" - Fusion Drive 3.5 GHz Intel Core i5 16GB RAM
I get a lot of lag on adjustments in LR 6 - the Mac "beachball" I also have LR on my MacBook Pro - 2012 8GB RAM that if I'm away I import the photos into and make adjustments but then export as Catalog when I get back to iMac - but working in the same version of LR6 on the MBP I don't get lag. And the iMac is so much faster than the MBP.
I have off the GPU on each too.
So it may not be LR, but something on the iMac hardware side. The 5k Retrina screen of the iMac make sit easier for these old eyes to edit for me than the MBP though.
Not sure there is a solution.
So with not much to do today I installed LR. First I used the Adobe Uninstall app. The I went through the two main directories of User/Library/Application Support/Adobe/LR and got rid of these LR files and also the MacHD/Library/Application Support/Adobe/LR. Then I also deleted the Adobe Preference plists in User/Library/Preferences.
Reinstalled LR and working smooth as silk - no lag. And call me crazy - the adjustments I had done and new ones on new photos now seem to be more accurate and crisp.Maybe not being frustrated with lag just makes me happy.
I DO NOT recommend deleting files if a person is not computer savvy and know what they're doing. I do, And I also have multiple backups of this computer that run hourly on two different hard drives and I also have just my Lightroom catalogs and Master photos backed up to an additional only LR hard drive backups.
Before doing any deleting I also moved out my installed Presets to the desktop for fast reimport to LR - instead of going to my backups.
DO NOT DELETE ANYTHING UNLESS YOU ARE BACKED UP AND KNOW ABOUT YOUR COMPUTER.
Reinstalled - sorry for typo above