You're right for the most part, fast charging can be detrimental, but it's not how fast you put power into the pack, it's how hot the pack gets in the process. It can take a lot more power without creating too much heat below 60% battery, but it's why charging speed extends so much after 80%. It has to reduce because it takes so much effort to top the battery off without creating too much heat. In the real world I'd rather charge at 120V all the time and just keep it plugged in every time it's in the garage, but in the case of a road trip that's not possible.This is true. If it's a leisurely trip, family vacation with ample time built in for travel, stops, etc. and more frequently family pit stops, I accept that one might just do shorter intervals, stop for the wife and kids to use the facilities, and juice up the battery over these 20 minute stops. When I travel to a destination, I'm more purpose driven to get there as efficient as possible. As the person doing the driving, I see a wasted 20 minutes at a stop, as being 30 miles I didn't drive. This, adds up. Stopping for ~20 minutes 10 times, is ~300 miles not traveled, which means another day of driving.
My rudimentary impression of "fast charging" is that it is less desirable and cuts battery life shorter than designed. So I'm under the impression a slow charge is better for the longevity of the battery.
To shorten it, heat damages battery packs, not speed of charge, but the faster the charge the more heat created.