I don't know anything about Macs, so I'm not sure if anything I suggest will help. You could try changing the preview size to Auto. Unless you have a high-resolution (4K) Monitor, try disabling the GPU option in Lightroom preferences.
Thanks for you reply. I already have the size set to Auto (2880px).
Are you importing all of your image into LR and culling from there? If so, try Photo Mechanic, you can cull there many times faster than LR. Then import the keepers from the culling process. That will save you all that time in LR making previews of photos you will not process.
How full is your hard drive? Even a SSD runs way slower as it gets fuller. I suspect it is your drive speed read and write. Run this test to confirm your hard drive speed. "blackmagic disk speed test" it is free and available from the app store.
To give you a idea of hard drive speeds my SSD on my Mac Pro is a 1tb with 700 gb free. its speed is 815MB/s write and 866MBS read. I have another raid card with two 1 tb SSD its write and read is 680MB/s. My drobo connected via usb 3 is 200MB/s read and write but my Mac Book Pro laptop with a 500Gb SSD with 165gb free is 173MB/s read and 114 MB/s write. I just cleared this drive to give more free space. When it only had 60gb free it was running only 80MB/s read and write.
My suggestion is keep your OS drive with as much free space as possible. Than use a external for image storage. I use a external drobo via USB 3 or Thunderbolt for storage of images. External with raid configuration will run much faster than just a single drive. I found these enclosures have great speed with new thunderbolt connection. /www.macsales.com upto 800MB/s read and write.
I do already use Photo Mechanic for adding XMP data, captions and for colour coding files. I then import all images in to Lightroom. I could only import selected images but there's often times I initially need to import images but then work on a second or third batch so importing all images seems to be a better way to work.
I will try importing selected images this weekend and try a different workflow.
Have you tried disabling the graphics processor in Lightroom? also, you may try and update the GPU drivers from the manufacture's website.