Some helpful links regarding plotting, optimizations, and efficiency.
The first phase generates all of your proofs of space by creating seven tables of cryptographic hashes and saving them to your temporary directory. Phase 2 back-propagates through the hashes, phase 3 sorts and algorithmically compress these hashes in the temporary directory while starting to build the final file and phase 4 completes the file and moves it into your final plot destination.
There are 4 phases when plotting Chia. Lets break these down
Phase 1 according to the link above creates 7 tables of cryptographic hashes and puts them in a temp directory. This phase is CPU intensive and is the only phase that takes advantage of multiple cores/threads. All the succeeding phases are single threaded. This phase also looks to use the most memory.
Phase 2 back propagates through the hashes, what this means exactly, I am not sure. This phase will use the most storage space. Phase 2 also seems to be a good time to start another plot if your plotting in parallel. Plotman by default starts another plot when a plot reaches phase 2:1 (:1 is a minor phase within phase 2)
Phase 3 sorts and compresses the hashes and starts building the final plot file. The total temp storage should decrease throughout the entire phase.
Phase 4 completes the plot file and moves it to the destination. It looks like if something happens with the destination you can manually move the 2.plot.tmp file to .plot and copy it to a destination file.