The heat generated while charging slows down the speed. Charge it in a well-ventilated or air-conditioned room for better results.
Instead of plugging it into your laptop or PC USB port, use a wall adapter rated 5W (1A at 5V) or higher. It will charge faster than the USB port on your computer.
Dust and lint on the charging dock or watch's back may affect the charging.
Turning off the watch while charging will keep it cool and speed up charging since there's no idle drain.
No. The watch supports standard 5W charging. However, the upcoming Galaxy Watch 5 is expected to feature 2X faster 10W charging.