WP Extremely Slow during only first ajax call

Howdy
since a couple of weeks something happened at my WordPress.
Everytime I load a page it takes from 7 to 14 seconds to load the first ajax call https://www.latribuna.sm. As soon as this first ajax is loaded the other ajax calls are very fast and the website loads quickly.

I f you check the website with Firebug in the section Net everything is more clear.

I tried everything:

• disabled plugins
• changed theme
• installed HeartBeat control
• increased memory limit
• used a Cache Plugin

and nothing worked.

It’s slow only the very first ajax call.