According to a new Apple support document, if your Apple Watch detects a charging experience that could be improved, a Slow Charger message will be displayed in the Battery section of the Settings app. Instances of slow charging are shown in orange, while fast charging speeds are displayed in green.
It all hangs on the meaning of "improved", doesn't it?
If they can detect the charging speed, why not display a number? Or some other visual representation of speed beyond slow/fast.
Even allow me to choose? Fast if needed, slow if in no hurry, such as overnight or some other reason it doesn't matter.