I upgraded to Lr6 from Lr5 and the develop mode is very sluggish compared to Lr5. It is slowing down my work flow significantly. I am disappointed with the performance of Lr6 as one of the reasons I upgraded was a faster develop mode as advertised by Adobe. My Mac is Retina early 2013, 2.6 GHz 8GB, HD graphics 4000 1024 MB
It's a tad difficult to establish the source of a problem or a potential fix when you've provided virtually no information regarding your computer hardware, the camera used for you photos, whether they're raw, TIFF, JPEG, etc. May be you could do so? In meantime, try switching off the GPU acceleration in Lightroom preferences. There issues with some GPU models.
I see you did provide the informattion on hardware, but it wasn't visible to me when I first read your post.
The Intel HD 4000 is one of the GPUs that are known to be problematic. Try switching it off.
It is better now, thanks!