Lattice plasmon cavity modes combined with optical gain can exhibit directional and tunable lasing emission at room temperature. However, the mechanistic details governing the dynamics before lasing action are not understood. This paper describes how the long photon lifetimes of lattice plasmon modes can be correlated with the ultrafast dynamics of lasing action and amplified spontaneous emission. Lasing from band-edge plasmons and amplified spontaneous emission from propagating plasmons showed rise times on the order of tens of picoseconds, during which inverted population in the gain was first generated and then followed by energy transfer to the lattice plasmon cavity for enhanced light emission.