We did some test, create 500000 random points, then render them via WicBitmapLayerOverlay, we found the GPU is used, but the peak value is when map initialize the points.
And pan or zoom after that won't make GPU get big changes.
After that we double check the code of WicBitmapGeoCanvas, and we make sure the code follow correct way to call D2D dlls.
So we think maybe the slow problem is not related with the part of D2D, but we also cannot make sure that, our developer will do further research about this problem.
If we have any update we will let you know, but for now please try to reduce the render shape number at the same time, that should be the quickly solution for it.