Fast charging generates more heat, and heat is the real enemy of battery chemistry — so the worry isn't baseless. But modern phones throttle charging speed automatically once the battery gets warm, which limits the damage far more than most people assume.
The habit that actually does more harm is keeping a phone at 100% for hours, especially overnight while it's warm under a pillow or charging case. Batteries age fastest sitting at full charge and heat, not during the twenty minutes fast charging takes to get there.
If you want to extend battery life meaningfully: charge overnight less often, keep the phone out of direct heat while charging, and don't worry much about the fast-charge brick itself.
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