You did nice to offload your Tableau Server on processing simultaneously of workbook extracts.
Also I just want to share some things that I did on my server.
Running tableau on minimum requirements well it would run but not the fast thing you expected.
Tableau Server workloads runs on the background and takes a lot for RAM.
So increasing RAM is better, 32 GB is minimum, but try to increase it as twice or higher.
Tableau Server also runs on disk of course, so you could upgrade your regular SATA HDD set up to SAS HDDs or if you could SSDs. Faster read and write means faster performance.
Also Tableau Server especially runs on Network, by using Gigabit Networks, 10/100/1000 could help
give your server network a higher bandwidth and data transfer rate from your server to your client and vice versa (of course partner it with a Gigabit switch) , than
using a typical Ethernet or LAN card that supports only 10/100 mbps speed.
These are some of things you could do to enhance your server performance.