Since both the Apple 10.5-inch iPad Pro and Apple 87W USB-C Power Adapter use the same Apple platform, we see how efficient charging the iPad can get from the Apple 87W USB-C Power Adapter that comes with the Apple 15" MacBook Pro (Late 2016) using USB Power Delivery technology.
While at 2% battery charge on the 10.5-inch iPad Pro, we use a Apple USB-C to Lightning Cable to directly connect the iPad with the Apple 87W USB-C Power Adapter. As seen from the Granite River Labs GRL-USB-PD-A1 test software, the Apple 87W USB-C Power Adapter initially declares a single USB Power Delivery source capability of 12 watts (5 volts @ 2.4 amps) after a hard reset.
The 10.5-inch iPad Pro goes ahead to request the 12 watts (5 volts @ 2.4 amps) charging power which the Apple charger then accepts.
Somehow the 10.5-inch iPad Pro again requests and establishes the same initial 12 watts (5 volts @ 2.4 amps) power contract with the Apple charger.
After going about multiple Discover Identity, SVIDs, Unstructured VDM and Attention requests, we can also see the 10.5-inch iPad Pro requesting 27 watts (9 volts @ 3 amps) which the Apple 87W USB-C Power Adapter also complies.
Looking at the power graph, the 10.5-inch iPad Pro when switched off at 2% battery initially draws around 13 watts (5.2 volts @ 2.5 amps). When reaching 110 seconds, the iPad is turned on and briefly stops pulling current for a couple seconds. It then resumes charging by pulling slightly lower current to charge at around 18.9 watts (9 volts @ 2.1 amps).
Compared to the 12 watts based charging that the 10.5-inch iPad Pro originally gets, the iPad can charge faster using the higher power supplied by the Apple 87W USB-C Power Adapter over USB Power Delivery.