See the Options: (http://en.wikipedia.org/wiki/Wikipedia:Tools/Navigation_popups#Options
To disable the previews, you can add one of two lines to your User:USERNAME/monobook.js, just below the rest of the Popup script.
(No data will be downloaded and the popups will just consist of a list of links.)
(Does not display preview)
Not sure what the difference is, as I like the previews myself, but try it both ways and see what you like. Hope that helps your experience.
(My favorite thing about Nav previews -- you can copy
from them! Brilliant when you're writing and article, and just want the proper spelling/capitalization for a link. Type in the most likely candidate, preview, hover, and if it's a redirect or dab page, copy the correct link from the preview box and paste it into the edit box. So fast!)