In order to reduce the calculation time of the algorithms, we will subset (or clip) the raster layer to the boundingbox polygon.
1. Now, from the main menu select Raster | Extraction | Clip Raster by Mask Layer.
2. In the Clip Raster by Mask Layer dialog, choose dem_reprojected for the Input layer. For Mask Layer, choose boundingbox. Check the box to Keep resolution of input raster and keep the defaults for the other options. Choose dem_subset for the Clipped (mask) output and click Run. Click Close when done.
Often you'll not have a bounding box shapefile. In that case you can choose Raster | Extraction | Clip Raster by Extent. Then you can drag a box in the Map Canvas and use that for clipping. While using that, make sure your on-the-fly reprojection is similar to the layer that you want to clip, because the map canvas coordinates are used by the algorithm.
3.
Now you can remove dem_reprojected from the layers list as we have done before for other layers that are no longer needed.
Watch this video to check the steps in this section: