I was working on a Foundation with Compass project and decided to put Grunt on it.
Compilation took 11.116s Running "watch" task Completed in 13.974s at Wed Dec 18 2013 13:53:05 GMT-0500 (Eastern Standard Time- Waiting... OK >> File "scss\_core.scss" changed.
I found out that Grunt compilation is incredibly slow with Compass. It was even more slow because I was using Compass sprites.
So if you plan on using Grunt with Sass, I suggest you use regular Sass.
You can create a new Foundation project with Grunt and Libsass using:
foundation new MY_PROJECT --libsass