It's a http/xml-based API, similar to the REST API that exists in V9, but undocumented. It's relatively easy to reverse-engineer if you use a proxy like Burp or Fiddler. The hardest part is how to encrypt the password. I actually rewrote the whole thing in Ruby and then in Java, to make it a little more usable. But it looks like you're doing some time-consuming operations; I would stay with the official tabcmd because some of the long operations are asynchronous and can get tricky.
But yes, it creates network traffic. Since you're using localhost, it should stay within the server.
I don't believe it is multi-threaded.
I'd do some timings to find out what has slowed down. I think it's more likely to be the rendering of the workbooks in Tableau Server, than tabcmd itself.