Whenever I increase texture bake to High or Ultra, Rhino will hang for hours. Render with Standard usually initializes immediately. Most complex element is HDR env, everything else is under 1 meg jpg bitmaps.
This happens if you use Rhino procedural textures on lots of objects. We can tell more if you can share a file with us. If you don’t want to post it here on the forum in public you can share it with us confidentially using our upload tool