Optimum photon lifetimes of vertical-cavity surface-emitting lasers (VCSELs) are shown to be target bit rate dependent. A comparison of the power budget in fJ/bit for optimized VCSELs operating at 25 Gb/s and 50 Gb/s is given. At 25 Gbit/s, the energy per bit ratio is lower than 100 fJ/bit presenting a 75% reduction as compared to the 50 Gb/s values. The cavity dip / gain peak detuning for maximizing temperature stability must be similarly adjusted to the bit rate as shown here. These conclusions are valid for any VCSELs, e.g. those emitting at 850 nm, 880 nm, 910 nm, 940 nm or 980 nm, presently investigated by us.