Parallel ploting

hello, i am trying to improve my plotting time.

Actually : 10-12 hours for 1 plot

I tried to run 4 ploting in parallel but I have one which goes very fast and the 3 others which hardly advance.

For history :

I have 2 cpu xeon E5-2470 ( 16 core by processor ) 2,3Ghz
60 slot for SAS disk with 36 disk SAS and 8 disk SSD

I use 8 raid 0 SSD for temp space ans 36 SAS for Permanently space.

I research à best configuration but he have lot of possibility.