The point is, that I still use the same host machine with the same kernel and the same virtual machines. There was even no difference with old (1.3.8 ) vbox extension and updated vbox additions.
So 1.3.8 was o.k. in terms of performance, 1.5.2 is not.
Do you have any idea which version between 1.3.8 and 1.5.2 may perform better?