dell precision 690 runs too slowly after sccm deployment

after several weeks using sccm over the enterprise network, all the 690 Workstations run too slowly, it takes 20 minutes to display the logon dialog box, and after that other 20 minutes to see the desktop.
i have updated the BIOS, and also reviwed documentation but i can´t find anything about it.
i wolu appreciatte your help.