I posted about this a while back and have some new info - heres a link to the old thread for reference -
GUI lag thread
Im now on pro tools 2020.5 and OS 10.13.6. Computer is Quad Core 2013 Trashcan Mac Pro.
I experience serious jerky GUI lag and audio drive "thinking" noise (typical hard drive read and write sound) when zooming in past a certain level ON MY DRUM TRACKS. It will zoom in or out about one level every five second. IF I CONSOLIDATE THE TRACKS THE ISSUE GOES AWAY. There are a couple of edits on the tracks, but nothing crazy at all. This issue has persisted across several versions of pro tools and at least Sierra and High Sierra. Other than that this setup runs like a dream.
I cloned my drive and moved to Mojave a few days ago, but that was a -6101 error disaster, so I reverted back to my very stable current setup.