Thanks.
We do have some curves with many points and I'll be looking forward to use the class you described in 6.1.
That, however, is not where our performance problem lies. Our custom item (a triangle mesh with lots of triangles) is the culprit. I think the main performance hit comes from a situation where lots of very small (color-filled) triangles are clustered together. If the same number of triangles is scattered evenly, there's no slowdown. I'll do some profiling and try and see if we can omit the rendering or filling some of them (or just replace them with dots) if all 3 points map to the same pixel.
Thanks again for the suggestions!