This means that you (or someone else) probably modified the core K2 template files instead of creating template overrides for K2, inside your Joomla template's /html folder. See this for more info on templating overrides: getk2.org/documentation/tutorials/templating-with-k2-and-the-concepts-of-sub-templates

As for the slowness you mention, does that occur in the backend or in frontend editing?

If you have sorting problems in the backend, are you perhaps using any other K2 plugin for extending K2's features? E.g. multi categorization?

Then check your settings for those sections that utilize core templates and have no overrides. Keep in mind that some category may inherit options from another, so you need to be careful when examining each setting.

As for performance, the new K2 does have the SQL queries in some cases. If nothing's changed in your server's performance that coincided with the update in K2, then double check for plugins that could interface with K2 in the backend. Again be very thorough. I've had people swear they had no plugins only to come back with a response like "in face I did have this one plugin that was outdated/broken...".