When i use k2 (2.6.9) Categories menu are too slow. When i click a joomla article or a k2 item (article) it works great. All the menus are "k2 categories". Click on anyone of them and you will see that it is really slow. If you click a k2 item it loads very fast (the site is optimized for low loading time). Here you can see it for yourself.
www.kavala-portal.gr
I think that speed issues for k2 and J3.x are very common i ve been searching the forum lately so an update the those issues (or an updated version of k2) maybe the best solution.
In the meantime, if you have any idea what could i do with the categories menu i would be very thankful!

Any help by admin, moderator or other joomla users would be appriciated :)

This could be related to a module, an underpowered server or an extension throwing some PHP warning.

In any case enabling cache will really help loading times.

I think that is not about server. I have a lot of websites on the same server and i have not any probem.
Also you can click on any k2 item or any Joomla item (the top menu of the website uses Joomla articels) and you will see that there is no problem with loading time.
I found something that may be the solution but i couldn't do it. Here is the link of the thread : forum.joomla.org/viewtopic.php?f=708&t=847976
Many people have reported website speed problems with J3 and K2.

Many of the issues reported there are about core articles as well.
Which other extensions are you using apart from K2?