Originally Posted by UsedPaperclip 500MB/s, which is barely more than USB 3.0.
PCI-E x2 would be 1GB/s
PCI-E x4 would be 2GB/s
It just multiplies as the x* goes up.
USB 2.0 supports a maximum transfer rate of 480 megaBITS per second. A single PCIe 2.0 x1 slot can support up to 500MegaBYTES per second. Also, with the way USB 2.0 handles duplexing, you'll never really see much more than 240megabits per second which translates into 30 megaBYTES per second.
You can in theory run 15 or so USB 2.0 ports at full speed off of a single PCIe 2.0 slot.
USB 3.0 specification supports 5gigabits per second transfer speeds, though I haven't yet seen a controller that could hit anywhere close to that. Most flash drives and external hard drives with USB 3.0 will max out at about 100 megabytes per second, meaning a single PCIe 2.0 x1 slot can comfortably support 5 USB 3.0 ports, and it would only become an issue if you started doing massive transfers from all five drives at the same time, which would be a very rare occurrence.