It may be or may not be anymore, but at one time not long ago it definitely was. When I cared enough to actually profile its performance, you could literally just move the mouse to the top corner to expose the dashboard, and back. Each time doing this, 4MB of memory gone for good. That particular bug has been fixed. JS engine memory leak. In fact, I invite anyone to look at the number of 'memory leak' commits in Gnome 3/Gnome Shell regarding its JS use.