PCI Express is the high-speed architecture that's used to connect many of the components in your Mac, such as the processor, graphics card, and hard drive You can think of PCI Express as an expressway that lets data move quickly and efficiently between these "locations." Because Thunderbolt is based on PCI Express, it offers a direct connection to the PCI Express bus, which is part of the reason it can offer such impressive performance.

In theory, it's blazing fast. A Thunderbolt channel can provide up to 10 Gigabits per second (Gbps) of data throughput--and each Thunderbolt port includes channels. Thunderbolt is also bi-directional, meaning it can transmit and receive data at the same time. Even with estimated real-world performance of around 8Gbps, Thunderbolt is many times faster than FireWire 800 and USB 3.0. It's also significantly faster than the eSATA connections available on many Windows PCs.

Of course, just as with previous high-speed interfaces, performance of each connected device will often be much lower thanks to the limitations of the device itself; for example, most SATA hard drives top out at 3Gbps, and even SATA 3.0 drives are limited to a theoretical 6Gbps. Similarly, an older, slower device placed in the middle of the chain can--if not connected properly--cripple throughput for higher-speed devices connected after it. (More on that below.)