Contour lines were created from the grid using the tool gdal_contour. I tried out different contour intervals and it turned out to work well with 1 metre:
gdal_contour -a elev grid.tiff -snodata 999999 shp/gridFull_1mCtrs.shp -i 1.0
The “no data” value set previously is crucial here – without it, the contours end up looking weird, as the tool will try to create contours for areas with no data.
As opposed to the grid, the contours only took seconds to generate. Here’s what they look like:
The images shows two different ways of layer blending between the contours and the layers below – left is “addition” and right is “multiply”. As opposed to “normal” overlay, those are ways to visually enhance contrast between this layer and varying colours of the layers below, see screenshot: