eSATA is a External Serial ATA interface. Serial ATA (SATA) is a commonly used interface for internal Hard Drives, and eSATA is an extension of that specification to be used with external devices.

If you're using a USB or Firewire external drive, what you're really using is a ATA or Serial ATA hard drive, and an external enclosure. The enclosure has a controller in it which translates the ATA or SATA protocol to USB or Firewire.

This translation causes some delay and there is also some overhead involved in the translations.