The GPU acceleration can´t be activated anyways with my old Mac.
i have a 13"macbook pro with OS10.11.5 and lightroom 6.6 was dam slow, so i shifted back to 6.3 which is better
but wanted to try guided upright tool in 6.6,, i guess i have to wait for software to improve either MAC OS or adobe lightroom
There are already dozens of threads discussing the very same issue in this forum as well as in the official feedback forum here: https://feedback.photoshop.com/photoshop_family/categories/photoshop_family_photoshop_ligh troom
You can roll back to 6.5.1 if you like. Here's how: How do I roll back to Lightroom 2015.1.1 or Lightroom 6.1.1? | The Lightroom Queen
Simply replace 6.1.1 with 6.5.1