It sounds more likely a feature request, there is no command line to collect the columns of a workbook at the moment.
You can an idea on the following forum : Ideas
Also if it's taking a lot of time to load the workbook :
1. Tableau Server :
a. Tableau Server is whitelisted in your Antivirus ? : Improving Performance by Using Antivirus Exclusions
b. Did you open the necessary ports in your Firewall ? : Tableau Services Manager Ports - Tableau
c. Do you have the Minimum Hardware Requirements ? Minimum Hardware Requirements and Recommendations for Tableau Server
d. Is Tableau Server configured properly ? (How much nodes, VizQL Server, Backgrounder for how much CPU ?) : Recommended Baseline Configurations
2. The workbooks are optimised ?