What version of Lightroom?
May i know what is the version of the Lightroom you are using.
Also what is the version of the mac os .?
Oh, Sorry for that, always the same...
Mac: IMAC 27" I7 3,4 GHz, 16GB 1333MHz DDR3, AMD Radeon HD 6970 1024MB
OS X El Capitan, Version 10.11.
GPU is turned off since I had the feeling that it helped a little bit.
CPU runs at 29% when using LR
Ram runs at 10% when using LR
Lightroom: LR CC 2015.2.1
That's probably your answer. Upgrade to 2015.3.