I installed and activated this plugin and it worked fine but the sliding was not smooth. It sort of hesitated at the start and end.
You may not experience this but for me it was because the ajax-loader gif was just a bit bigger than my line height. When it appeared or disappeared all the sliding text was moved a bit down or up.
I replaced the ajax-loader.gif with a different gif that was only 12x12 instead of the 16x16 default and the slide is now fabulous.
Might help someone...