Difficulty Merging calatogs with Lightroom Classic

When I travel I make  anew catalog on my portable hard drives, edit, rate, etc.  When I get home I merge these catalog together.  I have been doing this successfully for years. Ever since Lightroom Classic (v9) this has not worked well at all.  I am in the middle of merging a catalog of 5400 images with my Master Catalog and it has gone for 3 hours and only says it imported 500 images and has said that for over 1.5 hours. I find I usually have to quit merging and just import the images from the portable hard drive to get this to work now.  I am teaching a class "Lightroom on the Road" at the Florida Birding and Photo Festival in April and really wanted to show how this supposed to work by merging catalogs. Is anyone else experiencing problems merging catalogs?