WP lags big time- site loads forever…

Hi. What to do ? I mannage 5 sites, 1 of them takes ages to load, when it finaly loads it takes ages to login and again it takes ages every time I want to update a page. Im goin nuts, trying to get some work done in this laggy enviroment is a real nag. any ideas ?

PS- the other sites are all previous versions 3.2, and another thing when i tryed to auto intall plugins from browser thru admin panel, all plugins failed! I had to upload via FTP to get them working, is that a common problem ?

Regards Storm