Faster geometry rendering
When we need to visualize very detailed vector graphics wpf is pretty useless. Even the basic transformations, such as pan/zoom take a lot of time (using RenderTransform or GeometryTransform).
Pure Direct3D handles it very well, and that's what I would expect from WPF (the geometry is static and frozen).
I believe some of the greatest overhead comes from the per-primitive antialiasing. I suspect performance could be improved by offering greater control/flexibility over how antialiasing is applied, and what quality level is used.