Hi François and Shahil. I do experiment the same trouble. I run LrC 9.4 on a brand new iMac 2020,1 ( macOS Catalina, core i7, 64 Go DDR4, Radeon Pro 5500 XT 8Go). For instance, after selecting 1753 photos, the countdown takes 13 minutes to reache photo # 1136, and then hangs. Same thing with a catalog on my external SSD or on the HDD. Same thing after exportig a selection as a new catalog. I had a long chat with PRAWEEN KUMAR from Adobe support. He tried to change some settings with the permissions of user acount, but no effect. Finally, he said Apple have to help reinstalling graphic card driver. I was surprised ! Nevertheless, I contacted Apple support. Apple support asked me not to proceed with a migration from my old iMac, instead do a new install of the all system, and then a clean install of LrC. The same bug again ! I'm curently stuck here, Adobe and Apple rejecting the issue on the other partner.