Mount Feastus Oserstack Bug

Hello My PC is an 3.40 GHZAMD Ahtlon x2 with 12GB RAM Running Windows Professional 7 Service Pack 1 and my Graphic Card is a ATI RADEON HD 7470 1GB PCI-EX DVI DP LOW PROFILE  GRAPHIC CARD Yet I still get a massive lag on this mission despite having a powerful PC like this why is that? or is my PC not powerful enough?  It slows completely up and I get a mass of at least three attacks of 10's directed against my core