I would check your upload speed.
If it is maxing out - You WILL NOT get your max download speed.
(Every bit of info in needs error checking sent back, as well as the request for the info to be sent in the first place... If your upload is congested, your downloads will suffer)
Set a limit for your upload to be about 75% of your total available upload.
(Means you can browse the web, play games etc too)
THe speed of CUP makes no difference. (Except extreme's... Obviously a 66MHz 486DX isn't going to pull in much quickly!)