pre render north america very slow

I'm trying to pre-render the whole of North America from zoom level 6 to 18 using the below command

/usr/bin/perl ./render_list_geo.pl -l 28 -n 16 -z 6 -Z 18 -x -168.8 -X -50.8 -y 23.6 -Y 71.8 -m default

As per my calculations it should take around 2.8 TB disk space. The postgresql DB is on SSD drives while the tiles are being stored on SAS drives. When I had imported the OSM DB, it took only 61 hours for a planet import so I think drives are quite fast.

The pace of pre rendering is around 40 GB per day. At this pace it will take months for the pre rendering to finish. Please suggest how it can be made faster.

I am not sure whats the bottleneck because the server has fast drives and has 64 GB RAM and 32 cpu cores. The disk IOs are mostly un-utilized.

Pre rendering from zoom 6 to 15 is finished and right now its pre rendering zoom 16. Its the pace of pre rendering that worries me.

http://tools.geofabrik.de/calc/#type=geofabrik_standard&bbox=-168.8,23.6,-50.8,71.8&grid=1

If I pre rendered the above area from zoom 6 to zoom 18 then how long would it normally take. Please forgive me if I've asked any stupid question because this is the first time I'm configuring this server and have little knowledge of it.