I noticed that too, i'm glad you mentioned it. Q6600 running Chrome 15, everything was perfectly snappy until somewhere around then.
Maybe slideshare has some sort of architectural problem? Is it seeking through the slides linearly for every request? Maybe slides after a certain point get read from disk in multiple segments? Maybe at that exact point in time, the load on the server increased?
I hate slideshare requiring registration or facebook login before it allows me to download it in some decent format.