eSata plugged into Sata socket via adaptor cable very slow

eSata plugged into Sata socket via adaptor cable very slow

Why would an external eSata Drive (xSonic with 500Gb notebook Hard Drive) run ok on a Silicon Image based PCI-E to eSata adaptor at about 35MB/s, But then run really slow when plugged into NForce4 Sata socket via cable?