Plotting is both CPU and IO bound. Plotting can't fully take advantage of multiple cores. The workaround is to start N plots, each taking ~400GB of space and ~2 cores. On an 8 core server with 2x800 GB SSD, you can do 4 plots in parallel.

Assuming you only had 512GB of RAM, you'd only be able to do 1 plot at a time (since you need 400GB per plot). Plotting to RAM would need to be 4x faster, but it isn't since you'll be CPU bound. So it would be both slower and more expensive.

You need ~300Gb of 'temp' space per plotting process. So you could only do 1 plot on a system that with a fast SSD you could do 6 or more plots in parallel.