Why does capacitor charge slowly but discharge quickly?
When you charge the capacitor the 100k resistor limits the current so the voltage on the capacitor is:

$$v = V \left( 1 - \text{exp} \left( - \frac{t}{C \cdot R}\right) \right)$$

Where V is size of the input square wave and R is 100k.

The discharge current goes mainly through D1 and not through the 100k resistor. So the current is not limited by the resistor value and the discharge is much faster.

If you turn the diode around you should see a fast charge and slow discharge.