I would expect so.
Reasons:
Published speeds are often established in ideal circumstances.
There are two other devices involved (LG G3 and the external hard drive) that introduce their own hardware and software configurations. May or may not play well together.
Quality matters. A cheap, poorly made USB OTG may slow things up just as a matter of resistance and heat.
Yes. Expect a decrease in transfer speed.
The amount of decrease may or may not be noticeable or otherwise impact the required transfer speeds.