Here is that thread I was talking about…
Post 7 mentiones an InvViewMatrix bug. No idea if it affecting me here. The fact that it is happening on my mesh that isn’t moving is what makes me thing something is going on outside my control.
Apart from that I had some ideas for performance in BP with slices and thought that I could update the slices in sections over a few frames. The camera is unlikely to move that quickly so can update a few slices at a time. not sure how that affects rendering order for translucency (I’m hoping that the sections maintain their order that they were created in).