Another thing that people have forgotten over the years is that 5W bricks were (relatively and roughly) equivalent to the 20W chargers provided today. It's a pretty simple principle, but the smaller the battery, the less power that is required to charge it at a given speed. That being said, charging speed doesn't matter quite as much as the number of cycles that you put the battery through. There will be very little difference in battery longevity if you charge at 5W vs. 20W, particularly if other variables (such as heat) are controlled. I think that wireless charging is the worst thing that you can do for a battery health wise--it creates huge amounts of heat for the power that you are getting. Longer charging times at higher heat is invariably going to be worse than longer charging times at lower heat.