Yes, a lot of the slow down comes in the conversion of the radar picture into a 2d image (in RadarCalculation::render). In this the contents of scanArrayToPlot are used to re-draw a part of an image (irrlicht IImage) radarImage. This is later converted into a texture, and what you're actually seeing on screen is a 3d rendering of this plane mesh with the texture applied.
I've wondered about speeding this up by using plane mesh that has been generated with two triangles for each radar 'pixel' and setting the vertex colours directly. I suspect that this would be faster, but I haven't yet tried it.