he current sitefinity Taxon controls are...extreme. So if you link a content item to one single category, and you have 5000 categories in your system, the client renders out all 5000 into a hidden control on the page (for some reason)...serves to do nothing but slow down everything b/c your DOM is massive (jQuery parsing, markup size from the server, etc).
It's been an open issue for over a year, no fix in just yet, so until then here's a replacement that just renders out the tags nice and clean (with a seperator)...modify as desired :)
(Assumes the control lives here ~/Usercontrols/Widgets/Taxonomy/SimpleTaxa/SimpleTaxa.ascx)
Again, tweak as needed