According to code discovered in the latest beta of macOS Sierra, upcoming Macs may support USB 3.1 Gen 2 which would enable peripheral speeds up to a theoretical maximum of 10Gbps.
In a report from 9to5Mac, it was noted that a string in the Sierra beta mentions SuperSpeed+, a term reserved for Gen 2 ports. In fact, the report also specifically mentions the 10-gigabit speed cap, which is twice as fast as Gen 1.
Apple could be planning to include USB 3.1 Gen 2 in updated iMacs and MacBook Pros later this year, which may be announced sometime this fall. Support would likely come through Thunderbolt 3, which is not only cross-compatible with Gen 2 but offers native speeds up to 40 gigabits per second, and DisplayPort 1.2 connectivity.
Most people probably won’t see a difference between Thunderbolt 3 or Gen 2, but the new technologies simplify connecting 4k devices and bring speed improvements when backing up or sending files to an external hard drive.