A layer of gravel at the bottom of plant pots aids drainage.
Water does not move easily from a layer of finer-textured material to a coarsely textured one. The coarser the underlying matter, the longer a pot takes to drain. So skip the gravel. (To prevent the potting mixture from spilling out of drainage holes-which are a must-as well as to let air in and keep bugs out, use a piece of window screening, mesh drywall joint tape or a coffee filter in the bottom.)