If I recall, because it doesn't use the bus, but instead is a high speed serial system, it runs not only faster but uses less system resources.
Quote from Wikipedia:
PCI Express (
Peripheral Component Interconnect Express), officially abbreviated as
PCIe (or
PCI-E, as it is commonly called), is a
computer expansion card standard designed to replace the older
PCI,
PCI-X, and
AGP standards. PCIe 2.1 is the latest standard for expansion cards that is available on mainstream
personal computers.
[1]
PCI Express is used in consumer, server, and industrial applications, as a motherboard-level interconnect (to link motherboard-mounted peripherals) and as an
expansion card interface for add-in boards. A key difference between PCIe and earlier buses is a
topology based on point-to-point
serial links, rather than a shared
parallel bus architecture.
The PCIe electrical interface is also used in a variety of other standards, most notably the
ExpressCard laptop expansion card interface.
Conceptually, the PCIe bus can be thought of as a high-speed
serial replacement of the older (parallel) PCI/PCI-X bus.
[2] At the software level, PCIe preserves compatibility with PCI; a PCIe device can be configured and used in legacy applications and operating systems which have no direct knowledge of PCIe's newer features. In terms of bus protocol, PCIe communication is encapsulated in packets. The work of packetizing and depacketizing data and status-message traffic is handled by the transaction layer of the PCIe port (described later). Radical differences in electrical signaling and bus protocol require the use of a different mechanical form factor and expansion connectors (and thus, new motherboards and new adapter boards).
