Alrighty. Reporting back in after an hour of so testing the 10.5 iPad Pro, my config is 256GB LTE.
Battery life @ 44%
Testing gear:
- P3 Kill-A-Watt
- Apple genuine 61W charger
- Apple genuine 29W charger
- Apple genuine 1m USB-C to Lightning cable
I am sorry to report, the 61W charger only appears to deliver 18W (9V x 2A, measured 19.5W draw) from the 61W charger. Just to verify the charger is functioning as expected, I swapped out the USB-C to Lightning cable for a genuine 2m Apple USB-C to USB-C and connected the 61W charger to my 2016 MacBook, and got the expected 29W charging (30.5W draw) to the laptop. This is disappointing, because the charger can deliver 29W to a MacBook, but not an iPad Pro
I verified (again) that the genuine Apple 29W charger does supply a full 29W (~30.5W draw) to the iPad Pro, when the battery is <50%.
tl;dr
For maximum "fast charge" on the 10.5 iPad Pro, use a genuine Apple 29W charger + genuine Apple USB-C to Lightning cable, which delivers a full 29W to the iPad Pro 10.5.