Why did you remove you your Previews file? If you move or delete this file Lightroom will have to build a new Previews file. As you open a folder if the previews are not in the Previews file it will rebuild them.
Looking at your screen capture I would first suggest that you pause the "face detection" and then proceed to re-build your previews. As indicated in the earlier post if you remove or delete your preview files they will have to be rebuilt. The preview files must reside in the same folder as the Catalog file the one with the extension .lrcat ( LRCAT ).