I've heard some really good things about the Mate 9's charging speed, so that isn't entirely out of the realm of possibility. It'd be amazing if the Mate 10 Pro could keep up with the 5T, considering the fact that it has a larger battery.
Probably not much. The test was carried out using stock chargers and it'd be quite surprising if OnePlus didn't go through the hassle of ensuring their own chargers don't mess the device's battery.
Ever wondered how fast some of the best devices in the world right now charge? Pretty fast, if you consider the results of a test conducted on the OnePlus, LG, Apple, Google, and Samsung flagships for 2017. The LG V30, for example, was able to build up 53% worth of battery juice in just 30 minutes of charge time, and it still wasn't the fastest.