PCI Express bus
Link Speeds
Per wikipedia:
| PCIe version | Transmission rate | Encoding | Bandwidth (1x) | Bandwidth (16x) |
|---|---|---|---|---|
| 1.0 | 2.5 GT/s | 8b/10b | 0.25 GB/s | 4.0 GB/s |
| 2.0 | 5.0 GT/s | 8b/10b | 0.50 GB/s | 8.0 GB/s |
| 3.0 | 8.0 GT/s | 128b/130b | 0.99 GB/s | 15.7 GB/s |
| 4.0 | 16.0 GT/s | 128b/130b | 1.97 GB/s | 31.5 GB/s |
| 5.0 | 32.0 GT/s | 128b/130b | 3.94 GB/s | 63.0 GB/s |
These are the official specifications of the standard. "GB" is Gigabytes in base-10. As a rule of thumb, PCIe 4.0 can do 2 GB/s per lane, so 32 GB/s on a 16x slot. Some PCIe implementations may drive the links at a higher rate, for example the Extended Speed Mode (ESM) using 20.0 GT/s and 25.0 GT/s in CCIX on top of PCIe 4.0.
The transfer rates are relevant for e.g. GPUs connected to the CPU through PCIe (the common case for workstation/desktop graphics cards) and NICs.