As a followup, I picked up a
short cable with micro B female to C male from Amazon, and it also has the "charging slowly" issue, so I suspect there's just something inherent about going from micro B to C that limits the Nexus 5's charging rate - maybe it's due to USB OTG compatibility or something.
So for charging, it seems like we're stuck with A-to-C or C-to-C cables.