Presynaptic elements of cortical axons, where action potentials cause release of neurotransmitter, are sites of extensive functional and infrastructural demands. Here we report that the presence of K_v3 channels in addition to K_v1 at glutamatergic hippocampal mossy fiber boutons reduces the number of potassium channel proteins necessary to achieve fast action potential repolarization by about 60 %: from ~34/mm², if only K_v1, to ~14/mm², including ~15 % K_v3. The approximately 10-fold higher repolarization efficacy per channel of K_v3.1/K_v3.4 containing heteromers results from a higher steady-state availability, a better recruitment by the action potential, and a larger single channel conductance compared to presynaptic K_v1 channels. Thus, the functional properties of K_v3 channels at mossy fiber boutons are tuned both to ensure brevity of presynaptic action potentials limiting glutamate release, and at the same time to keep infrastructural costs low.