I had this issue after upgrading as well. It was very frustrating. Apparently this has something to do with the way Adobe is allocating resources. I was using a Microsoft surface Pro 3 with 8 GB of RAM and the i7 processor. In the past, it was certainly enough to do photo merging. After the upgrade, like you, I was experiencing the frustration of getting stuck.
I tried everything to no avail. However, I finally started shutting down every program that I had except for Lightroom. I shut down my browsers, my music in the background, and pretty much everything else. After freeing up some resources, suddenly it all began working.