Fast charging is marketed aggressively and worried about constantly. The effect is real, modest, and smaller than several other things people do not worry about.
The mechanism
Pushing current into a battery quickly generates heat and stresses the electrode structure. Both accelerate the chemical degradation that reduces capacity over time.
Heat is the dominant factor. A battery charged fast in a cool room degrades far less than one charged slowly in a hot car.
What phones already do
Modern charging is managed. The phone draws peak current only in the lower part of the charge range and tapers substantially as it fills, precisely to limit stress.
Temperature is monitored and charging slows or stops if the battery gets too warm. That is why fast charging sometimes is not fast.
What matters more than charging speed
Temperature generally. A phone in direct sun or under a pillow ages faster than any charging habit causes.
Time spent at full charge. A battery held at 100 percent sits under stress, which is why charge limits at 80 percent exist and why they are the single most effective setting available.
Deep discharges, which are harder on the cell than topping up.
What to actually do
Use the 80 percent limit if your phone offers it – both Pixel and iPhone do – and disable it before a day you need full capacity.
Charge overnight with optimised charging enabled, which holds at 80 and completes before your alarm.
Take a thick case off if the phone gets hot while charging.
What not to worry about
Using fast charging occasionally. The convenience is worth the marginal cost.