Anand Lal Shimpi from AnandTech comments on a new I/O design:
Today, Thunderbolt exists as four 10Gbps channels – two upstream and two downstream. Each channel however is fully independent. Although PCIe and DisplayPort are muxed from the cable perspective, you can only send one or the other over each channel. That limits max performance for a single storage device to 10Gbps (minus overhead), and it similarly limits the max display bandwidth to 10Gbps as well. The latter is insufficient for 4K video (~15Gbps depending on refresh rate). If you bypass Thunderbolt and just send DP 1.2 over the cable it’s not a problem, but if you want to enable mixed use cases where you’re driving both 4K video and high-speed storage over the same cable you’re going to need another solution.
Read more at: Intel’s Thunderbolt 2: Everything You Need to Know