There are several resources available for evaluating performance of Tableau.
There are also several different factors that can impact performance. Including the resources available on the data source machine and the machine executing the extract. The network the system is running on can also factor into the performance of the extract. So to comment on whether or not this is expected is difficult.
There are also some good tips and tricks in this blog on Tableau.com: Tableau Data Extracts - Tips, Tricks and Best Practices | Tableau Software
Hope this helps!