The latest BIOS and Thunderbolt firmware update as of March 2 fixes the battery recharge rate of the ThinkPad X1 Carbon. If purchasing a unit from a reseller, the system may be outdated and in need of a manual update.
https://www.notebookcheck.net/PSA-Update-your-Lenovo-ThinkPad-X1-Carbon-BIOS-or-be-prepared-to-wait-6-hours-for-a-full-charge.459335.0.html
A slow trickle charge may actually be useful in extending the battery's overall life.
That being said, 6 hours is a bit much.