Update: It appears this may be a function of the 1TB drives fitted to both 13- and 15-inch models. The reason for this isn’t yet clear: it may be the drives used offer greater bandwidth.
Benchmark tests by French site Mac4Ever show that the latest MacBook Pro 15 is delivering SSD read and write speeds in excess of 1GB per second. The site repeatedly achieved these speeds when Apple claims only “up to 775MB per second.”
The MBP 15 is able to achieve these speeds because it has a 4-channel PCIe connection to the SSD, in contrast to the 2-channel link on the MBP 13 and MacBook Air models, though from some reader reports this may be the case only on models fitted with 1TB drives …
PCIe is a high-speed serial link used only on the latest Macs, replacing the slower SATA connections of earlier machines. SATA 3 is good for around 600MB/sec, which was more than fast enough for hard drives but not capable of keeping up with the speeds of modern SSDs. PCIe 3.0 is capable of a theoretical maximum of 8GB/sec, allowing plenty of spare capacity for faster future generations of SSD.
Per lane (each direction):
- v1.x: 250 MB/s (2.5 GT/s)
- v2.x: 500 MB/s (5 GT/s)
- v3.0: 985 MB/s (8 GT/s)
- v4.0: 1969 MB/s (16 GT/s)
So, a 16-lane slot (each direction):
- v1.x: 4 GB/s (40 GT/s)
- v2.x: 8 GB/s (80 GT/s)
- v3.0: 15.75 GB/s (128 GT/s)
- v4.0: 31.51 GB/s (256 GT/s)
It seems likely that Apple was keeping things simple by citing the performance available across the range rather than noting the far higher speed of the top-of-the-range model.