Maya has an interesting workaround for speeding up their retopo operations (i.e. for hit-testing a mesh with millions of polygons). In essence, they simply store the depth map on the GPU and hit test against it (as seen in this “handy hints” video on YouTube.
Retopology isn’t something I’m having huge issues with at the moment, so this isn’t a complaint from me. I have been talking to and watching sculptors that have… “some issues” with performance regarding Blender when it comes to this things, so thought I’d mention the idea while I had it.