Why does charging takes longer than discharging in capacitors

The voltage on the capacitor changes faster when there's more current flowing in (or out), and slower when the capacitor is bigger:

rate = IC volts per second.

Here I is the current and C is the capacitance. If current flows in, the voltage increases. If current flows out, the voltage decreases. It works with negative signs on the rate and the current: current flowing out is written as negative.