So Fotis now has posted a K2 2.6.5 news and I don't have facebook, yahoo, aol and hotmail to comment on the article (he disabled the default K2 comment system for some reason, but that's ok, because he wasn't approving comments anyway) I'm going to ask the question here.

In the source code of the system/k2.php file I found this:

/ Define the default Itemid for users and tags. Defined here instead of the K2HelperRoute for performance reasons.
// UPDATE : Removed in K2 2.6.5. All K2 links without Itemid now use the anyK2Link defined in the router helper.
//define('K2_USERS_ITEMID', $componentParams->get('defaultUsersItemid'));
//define('K2_TAGS_ITEMID', $componentParams->get('defaultTagsItemid'));

I have bolded the important part, performance reasons. Can you elaborate a bit more? Does enabling the duplicated tag/user pages options in K2 2.6.4 result in performance issues? How many queries are executed compared to the new method in K2 2.6.5?

My next question is also releated to performance issues. In K2 2.6.4 the description for the advanced SEF settings is as follows:

Enabling this option will modify all K2 URLs making them more SEO friendly. However this option will add a significant performance hit for websites with more than a few hundred K2 items in them. Use with caution. This option is disabled by default.

This sounds as if the new URLs are stored in the database just like sh404 sef URLs. Is this the case? Can you elaborate more?