You mentioned that you've tried reading it on 4 different computers, I suppose you were using a Pro Duo reader connected to a USB port on those 4 different computers? What Pro Duo reader were you using? It could be a limitation on that Pro Duo reader you have.
Btw, the sustained read speed isn't so critical as the sustained write speed. The sustained write speed spec is critical for keeping up with realtime video camera recording throughput requirements. In this regard, have you ever noticed a problem with your Pro Duo memory card not keeping up during video recording?