wordpress loading slow > 45 s per page

i’m using wordpress in various blogs since a couple of years and worked fine in almost every version. since 10 days i face my wordpress installations to load very slow (>45s per page) so that you can say that the blogs are quite dead now.
i looked in the forums but could not find any post that had the same problem. one of the blogs where i’m testing to fix is

https://www.cruiseguidetraining.com

i had it on v. 2.5.1 but upgraded to 2.6.5 after the page loading problem started. i tried everyting from disabeling all plugins to changing the template to the default one. nothing worked so far.

same problem is still on other blogs on 2.5.1 and even wordpress mu.
another site on the same server running with joomla has no problems.

thank you for your help.

axel