We studied the transient behaviors of current-injection quantum-dot microdisk lasers at room temperature. Unique optical responses were observed, including the suppression of relaxation oscillations and fast turn-on. With the help of rate-equation modeling, the suppressed relaxation oscillations are attributed to the enhanced spontaneous emission factor in microdisk lasers. Short turn-on time, around 1 ns without pre-bias, results from the reduced carrier lifetime caused by the Purcell effect and increased nonradiative recombination rate due to higher surface/volume ratio. With short turn-on time, a large-signal direct modulation experiment at 1 Gbps is demonstrated. Modal transient behavior was also investigated under various temperatures from 100 to 300 K. Both of the transient lasing and steady-state lasing from side modes are suppressed at temperatures higher than 250K. Therefore, the quantum-dot microdisk lasers show the potential of single-mode operation under high-speed modulation at room temperature.