@dankcushions Thanks. I prefer shaders, as the zFast gives you a nice touch of bilinear with scanlines. I found the curved version was slowing things down, but when I went with standard, it was fine. I guess that little extra processing step to warp the edges was just a little too much. When I remove that, things work pretty well (and look great).
Please do not post a support request without first reading and following the advice in https://retropie.org.uk/forum/topic/3/read-this-first