There are different variations of exocytosis that might affect the rate of exocytosis at the axon terminal. One such variation is termed "kiss and run," which occurs when the neurotransmitter-containing vesicle fuses with the presynaptic membrane just enough to release neurotransmitters. The vesicle then pinches back into the axon terminal. This reduces the need to produce new vesicles in preparation for a new action potential, and so might increase the speed of synaptic transmission. 

Brandeis University

Since exocytosis describes the release of neurotransmitters from vesicles with the help of enzymes during synaptic transmission, it is understandable that the following three things affect the rate of exocytosis: 1.) The rate of production of neurotransmitters, 2.) The rate of production of vesicles, and 3.) The activity of the neuron (i.e., the frequency of action potentials). Normally, the axon terminal contains enough neurotransmitters and vesicles to prepare itself for an action potential. These neurotransmitters and vesicles are produced in the cell body with the help of enzymes and transported to the axon terminal. 

The Rate of Exocytosis

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

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