I have the Belkin BoostUp Fast Charger (stand-alone) and have had slow-charging issues as well. I've also seen it using the Apple cable and Apple charger, but only after trying the Belkin charger. I just received a replacement charger from Belkin - the new charger has the same issue.
Question for others - does the position of your Apple Watch (flat versus nightstand) impact? I have a theory that the issue is based on the nightstand mode, but I haven't finished testing yet.
Question for others - does the position of your Apple Watch (flat versus nightstand) impact? I have a theory that the issue is based on the nightstand mode, but I haven't finished testing yet.