Basically, Tableau takes your calculations and stores the fields in the extract so it doesn't have to re-compute them every time the view gets opened.
So if you're building dashboards and creating a lot of calculated fields it's a good idea to remember to optimize the extract as you go along.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgSaPfLzat7dt0JQaGZCtvN3NYXhz5P_R7My42SBATy2FW1TurkqXDnO3A5lep0norC1oAV6L6PRPs5bA0jAZf9VmgMkx7sxLZUMx5vVN8fRNRl7co2sftDU3WaPPq0q1YMN6BcUTB2bmc/s1600/Optimizing+Extracts.png)
Cheers!