PCIe 3.0 introduced , which PCIe 4.0 retains. This adds only 2 bits of overhead for every 128 bits of data. The efficiency skyrockets to roughly 98.5% .

The PCI Special Interest Group (PCI-SIG) adheres to a strict doctrine: .

For storage, the spec was a revolution. Storage controllers are bottlenecked directly by interface speed. PCIe 4.0 allowed SSDs to jump from sequential read speeds of 3,500 MB/s to 7,500 MB/s. This drastically reduced load times in gaming and drastically improved queue depth performance in data science and video editing workloads.

The PCI Express 4.0 specification is not merely a document about faster speeds; it is a document about overcoming physical limits. It represents the boundary where NRZ signaling (Non-Return-to-Zero) began to hit a wall, paving the way for PCIe 5.0 and 6.0 to adopt PAM-4 (Pulse Amplitude Modulation) encoding.

If PCIe 4.0 had stuck with the older 8b/10b encoding, the doubling of the clock speed would have been partially negated by the overhead. By maintaining the efficient 128b/130b scheme, nearly all of the raw bandwidth increase translates directly to usable data throughput.

The headline feature of the PCIe 4.0 specification is the doubling of the transfer rate compared to its predecessor, PCIe 3.0.