I decided to conduct a test comparing SONY MS Duo Pro 1GB vs SONY MS DUO PRO HIGH SPEED (both MS had the small PSP sticker on the pakaging).
I connected my Jap 1.50 PSP via USB to my PC and transfered a 753MB file. PC is not that high spec, AMD 2100, 768mb memory & USB 2.0 ports.
SONY MS DUO PRO 1GB - 5:17 min
SONY MS DUO PRO HIGH SPEED 1GB - 2.32 min
So whatever people say about the PSP not having a High Speed bus to take advantage of HIGH SPEED DUO it is not completely true.
It might not have the high speed link, but HIGH SPEED DUO certainly makes a difference.