Slow charging is actually better for battery longevity than fast charging. For example, my phone will limit it's charging speed at night when I have an alarm set, because it assumes I won't need it until my alarm goes off, and the slower it charges the less damage is done to a battery.
In general, lithium batteries are damaged by heat. Rapid charging creates extra heat, so it's worse for the battery. Manufacturers work hard to minimize the damage of quick charging, but it's still somewhat better to slow charge when you have the time.