Cool painting!
I would lower the resolution of the original. That way you can mechanically trace the overall shapes for basic placement and then refer to the original as needed. But as an artist you can paint in all the fine stuff. I haven't done this but you may also find that having the original open in a simple picture viewer and flip flopping or having the programs open side by side might work. But I've not tried it.
Second idea, you can keep the resolution up, but chop your original into quarters (ie smaller pieces that show what you need to know) and import and discard them as needed when you have a particularly challenging area where you want to work out some details.
I don't know everything about your setup, but am guessing that the resolution of the reference is really over-taxing your computer's power. And my suggestions are merely hacks until you get an answer from people that know what the program does better than I.
"Not a bit is wasted and the best is yet to come. . ." -- remembered from a dream