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.
You could try enabling GPU acceleration in preferences. I had the same problem with SP3 trying to do HDR from three 20MP files, just stuck at generating previews and not taxing any CPU or SSD resources in task manager. Enabled the GPU acceleration and now it works as before.