Snow Leopard performance issues?

I have a friend who recently installed Snow Leopard on a 21" Mac. Now she is having all sorts of performance issues. Internet periodically not functioning. Slow opening of programs, particulary when another is open, and general slowness of the whole system. I myself never had this problem when I loaded SL, so I am mystified as to what is happening. Did a search but came up with no real useful information.
Any clues?

Ken