As the project requires, I built virtual raster catalog merging two raster DEM files. Because I don't want to use the whole surface of the new Virtual Raster, I went on to clip a desired rectangle. When I do this, QGIS is taking forever to complete the process (forever meaning it never completes it) and after a while, I have to close the whole program and restart it. I found this problem to be more often in the latest versions of QGIS (I'm using the latest, 2.14.2 version). ps. I checked the newly created file while it was still processed and the file size was ridiculously large (I mean gigabytes, while in the past the output was with smaller file sizes).