WP fresh install, very slow page load

Hi there!
I am very confused! I have installed a fresh wp on a sub-domain on a dedicated hosting environment (w/ plesk panel) and page load time on firebug counts 15 – 20s(!) for back-office and 30% less for front-office.
Main domain works fairly, on a Joomla cms.

Also, I have many other domains with wp’s and other sub-domains too, with load time up to 3.

There is a strange DNS records behavior for the specific sub-domain. Maybe the DNS check tool that I used is unreliable, but it shows the exact same results for other normal functioning domains too.

Any ideas????