I do a lot of timelapse sequences and after exporting a reasonably large number of files (500+ to JPEG because Premiere Pro won't handle raw files) and executing a group command on those images the Lightroom application becomes very unresponsive. Your one option is to close the application down. At that point a view on Task manager shows that the closed application is still live in the background (usually running with about 1GB of memory) and can take up to an hour to fully close down. Note that if you've attempted a panorama the preview window appears about five minutes after application close. I'm running on Windows 10 with 32GB of memory and an Nvidea GeForce GTX980 video card.