At first, I'm not sure about USB OTG is the same as USB 2.0 or USB 3.0 in the capability to reading data or transfer files from external hard disk. I'm using WD Elements 500GB of disk capacity for this testing, one of product from Western Digital Corporation. Maybe, if from the flash disk I have no doubt.
Well, the results of this testing with the 548MB sized folder contain 357 files. Total time required is 10 minutes 47 seconds with average transfer speed of 1MB/s.
My conclusion is notthing different between the USB OTG capabilities with USB version 2.0. and works well as USB in general.