Two more points.
- Decreasing Min Shading Rate below 1 can help.
- You probably want to set Diffuse_Value = 0 and Specular_Value=0 and plug the image into Alternate_Diffuse. Only plug it into Diffuse_Color if you want your lights to light it and shadows to fall on it. For example, if the image is of a stage curtain, then light it. If it's an outdoor sky/trees/ground then don't light it - use alt diffuse. That channel is a straight passthrough to the render. The Diffuse_Color channel goes through an internal Diffuse node first.
Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)