I have to layout a 16 x 12 ft (size of tarpaulin) document in Photoshop, but when I change the resolution to 300 it's too large and it takes time to render the layout. What should I do?
Talk to the printer (the person not the machine). They will likely tell you what scale and/or resolution to work at.
You most likely do not need to work at 300ppi at full scale. It is more common for large format printing to work at a smaller scale or lower resolution (refer to point 1.)
Read through the answer here: I need to print an image at a certain size. What dimensions and resolution should I use?
They will likely print at 200 or 100dpi, depending on the type of print format. I would call the printer and ask for their file specifications as the person stated above.