Sounds like you network card on the pc is not a fast one or that the router can not handle fast connection.
Old network cards and router might only be able to handle a connection of about 10meg (was like that about 5 years ago for me on my old pc).
You might want to use the task manager and mounter you connection speed as you use the ps3 to download a movie file from your pc to the ps3, and see if the speed is reaching its cap or not.
You might want to check your router and find out what kind of speed it can handle.
All I know is it worked for me and I was able to download a 1.5gig movie file to the ps3 in less than 10mins and that was going at about 85Mbs
In theory the router should be able to handle 20Mbs, upload from my pc to the ps3 of 330 megabytes took over an hour yesterday although I've got a 512kbs upload. (the 20mbs download and 512kbs upload are the limits due to my connection package, expensive for what I get but not easy to find cheaper as yet in Belgium)
I've had downloads from the internet of over 1.5 megabytes per seconds, but never all that much faster.