I was starting to run out of space on my iMac’s internal 512GB SSD. So moved the Lightroom folder that has the catalog and previews to an external 1TB SSD connected via TB3. I got back 256GB of space on the internal SSD and now have plenty of space to build all the 1:1 previews for the ~100K images in the catalog. I changed the default catalog location in Preferences and verified all is well. The external drive is included in TM backups. So if you have space constraints on your boot drive to handle your Lr catalog and previews, you might consider using an external fast drive on a fast transport. You can not notice any performance lost. I have not noticed any performance hit. Your milage may vary.