If ReadPathAsPathGeometry is false (default) all the paths are read as StreamGeometry. The svg file is read much faster and also has better performance when drawn in WPF.
If ReadPathAsPathGeometry is true all the paths are read as PathGeometry. It takes longer to read the svg file. Also the performance in WPF is not as good as with StreamGeometry (ReadPathAsPathGeometry == false). The advantage of PathGeometry is that it can be modified.