charging efficiency

I have a 2016 Model S that I have charged mostly at 120 V. I'm approaching 90,000 miles with little or no loss in battery capacity. I'm wondering if charging slower helped. For example would the slower flow of electrons result in less side reactions the degrade the battery?

I am trying to decide whether to shell out for a 220v outlet in my garage for my Model 3. Does a charge cost less when it's 220v than when it's 120v? If so, how much of a saving is it?

I installed 2019.32.12.2 about two weeks ago. Eight days ago I departed for a long trip. In the past my Model 3 has lost about 0.5% per day ( my prior Model S was typically about 1%). This time after eight days I have precisely the same charge I had when I left. I did plug in the car but I set...