Not scientific by any measures, because I really don't care, but when I use the charging-only ports on my USB 3 Hub, my 5X says it's charging rapidly. If I plug it into one of the ports that is recognized by my PC (all but the bottom two), it says charging slowly.

This is the one I bought: https://www.amazon.com/dp/B00FR795WA/ref=cm_sw_r_awd_5xbowb1V1E2Y5

EDIT: Forgot to mention, but I'm obviously using an A to C cable.