Previewed at WWDC, launching in the fall.
Thunderbolt 3 Ports on Right Side of 13-Inch MacBook Pro Have Reduced PCI Express Bandwidth
According to the document, while all of the ports on the 15-inch MacBook Pro and the 13-inch MacBook Pro without a Touch Bar offer full Thunderbolt 3 performance, only two of the four ports on the 13-inch MacBook Pro with a Touch Bar support Thunderbolt 3 at full performance.
The two ports on the right side of the machine have Thunderbolt 3 functionality but with reduced PCI Express bandwidth. For that reason, Apple recommends plugging higher-performance devices into the left-hand ports on that machine.
Late-2016 MacBook Pro models vary slightly in the data speeds they provide to each Thunderbolt 3 port.As for USB, all of the USB-C ports on all MacBook Pro models offer USB 3.1 Gen 2 (10Gb/s) transfer speeds when connected to a USB accessory.
MacBook Pro (15-inch, Late 2016) delivers full Thunderbolt 3 performance on all four ports.
MacBook Pro (13-inch, Late 2016, Four Thunderbolt 3 Ports) supports Thunderbolt 3 at full performance using the two left-hand ports. The two right-hand ports deliver Thunderbolt 3 functionality, but have reduced PCI Express bandwidth.
MacBook Pro (13-inch, Late 2016, Two Thunderbolt 3 Ports) delivers full Thunderbolt 3 performance on both ports.
Other interesting tidbits in the document include the fact that six devices can be daisy-chained to each Thunderbolt 3 port on the MacBook Pro, and only one power supply can be used to charge the machine. You can attach multiple power supplies, but it's only going to draw power from the one that provides the most power.
Power supplies that exceed 100W have the potential to damage the Macbook Pro, and accessories like the USB-C VGA Multiport Adapter or the USB-C Digital AV Multiport Adapter can only provide up to 60W of power, which will offer slow or delayed charging in the 15-inch MacBook Pro. Apple recommends charging the 15-inch model with the power supply it ships with.
Apple also outlines powering attached devices with Thunderbolt 3 ports. The 15-inch MacBook Pro and the 13-inch MacBook Pro with Touch Bar can power two devices that use up to 15 watts and two additional devices that use up to 7.5 watts. The 13-inch MacBook Pro with no Touch Bar and two Thunderbolt 3 ports can power one device that uses up to 15 watts and one device that uses up to 7.5 watts.
If you're planning to purchase a new MacBook Pro and are confused about which adapters you're going to need so it will work with your existing equipment and accessories, Apple's support document is a good reference to check out.