I am trying to work with a large image (20 x 30 inches 300 dpi) in Painter 2020 and I am having a very difficult time. Painter keeps crashing. And when it doesn't crash, it is painfully slow. Is this program not meant to handle such large images?
By my estimates that is about 6000 x 9000 pixels, so yes it's a large document but Painter can go even larger on a powerful computer. How many layers are you creating? Are you using Watercolor and/or Thick Paint?
Maybe there's something else leading to the instability and performance issues? Are there specific steps that seem to cause the slowdown or crashes?
Often you can paint on a smaller image and then up-resolution it to final size. This works for paintings that don't have small detail because up-sizing generally adds softness but paintings are usually soft.
Alternatively you can add small details to the upscaled canvas and do the soft work on a small canvas.