By definition slew rate of a circuit is rate at which a circuit can charge and dischare capacitance. This capacitance may be external capacitor CL or Cg gate capacitances of transistors connected to this circuit.
Normally a digital circuit during switching must charge or discharge CL or Cg at faster rate, and this charging rate depends on output current of the circuit.
capacitive loading occurs when this output current is insufficient to drive load capacitances CL and one or more gates connected to original circuit as a result slew rate of the circuit decreases and circuit becomes slow(takes more time to charge capacitors connected to circuit).