MAJOR EDIT:
I have conducted my own test now. The setup:
iPad Pro 2018 12.9"
USB-C to USB-A adapter cable
An old, cheap USB-A 3.0 four port hub
Sony UHS-II USB-A SD card adapter
Sandisk Extreme Pro 64GB 300MB/s card exFAT
G-Drive 5400 RPM 2TB External Drive exFAT
Samsung T5 2TB SSD exFAT
I...