You are not going to change the speed with a USB to USB-C dongle. The transfer speed will only go as fast as tthe slowest port will allow. It will transfer at the USB speed not at the USB-C speed. Thanks.
The USB port is a hardware upgrade.
If you have a USB 2.0 or USB 3.0, connecting USB-C dongle will not boost the speed, it will make the USB-C device compatible to connect with your rMBP.
As far as I know, the speed at which iPad Pro can transfer depends upon the predefined speed of lightning port & the USB cable provided from Apple. I have read somewhere that the iPad Pro port is USB 3.0 compatible but the cable which is provided can handle USB 2.0 speed.
I would suggest if you confirm from Apple Care whether your iPad Pro support USB 3.0 port if that so then you can ask them from where you can get a genuine Apple USB 3.0 cable for your iPad Pro. This might increase the transfer speed.