Some of users say that K2 category page (view = itemlist) is so slow loading with K2 additional categories plugin.
An advice to optimise it very simple and addressed to developers: just add indexes for fields catid, itemID in _k2_additional_categories table.

So in the table should be:


What query was most slowest:
Screenshot of First query
And the second same query wich just ask database for amount of founded records

What we have after adding indexes now:


And the last query we can optimise by this way.
Open file components\com_k2\models\itemlist.php, find out function getTotal() and change it to:

That's all!

Nice :)
Thank you so much for your efforts.
Did you send this to the devs of that extension?