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)

FindThese.ascxView on GitHub
<!-- FIND AND REPLACE THESE IN YOUR TEMPLATES -->
<div class='sfitemHierarchicalTaxon sfitemTaxonWrp'>       
    <sf:SitefinityLabel runat="server" Text='Categories:' WrapperTagName="div" HideIfNoText="true" CssClass="sfitemFieldLbl" />
    <sf:HierarchicalTaxonField runat="server" DisplayMode="Read" WebServiceUrl="~/Sitefinity/Services/Taxonomies/HierarchicalTaxon.svc" TaxonomyId="e5cd6d69-1543-427b-ad62-688a99f5e7d4" Expanded="false" TaxonomyMetafieldName="Category" BindOnServer="true" />
</div>
<div class='sfitemFlatTaxon sfitemTaxonWrp'>       
    <sf:SitefinityLabel runat="server" Text='Tags:' WrapperTagName="div" HideIfNoText="true" CssClass="sfitemFieldLbl" />
    <sf:FlatTaxonField runat="server" DisplayMode="Read" WebServiceUrl="~/Sitefinity/Services/Taxonomies/FlatTaxon.svc" TaxonomyId="cb0f3a19-a211-48a7-88ec-77495c0f5374" AllowMultipleSelection="true" TaxonomyMetafieldName="Tags" Expanded="false" BindOnServer="true" />
</div>


<!-- REPLACE WITH THESE -->
<sf:SimpleTaxa runat="server" TaxonomyType="Category" Title="Categories:" />
<sf:SimpleTaxa runat="server" TaxonomyType="Tags" Title="Tags:"  />

Again, tweak as needed