The kagome compound CsTi${}_3$Bi${}_5$ serves as a natural counterpart to the extensively studied CsV${}_3$Sb${}_5$, offering an ideal platform for directly comparing their emergent physical properties. Here, we employ ultrafast pump-probe spectroscopy to investigate the nonequilibrium carrier dynamics and collective excitations in CsTi${}_3$Bi${}_5$. The quasiparticle relaxation consists of several decay components, among which the fast sub-picosecond decay is governed by the electron-phonon ($e$-$ph$) coupling. The extracted coupling constant of approximately 0.413$\pm$ 0.007 is smaller than that reported for CsV${}_3$Sb${}_5$. This weak coupling strength may be one factor contributing to the absence of a charge-density-wave (CDW) order in CsTi${}_3$Bi${}_5$, even when its van Hove singularities are tuned to the Fermi level, suggesting that $e$-$ph$ coupling may play an important role in stabilizing the CDW state in CsV${}_3$Sb${}_5$. In addition, a coherent acoustic phonon mode with a frequency of 0.012 THz is identified, corresponding to a sound velocity of $v_s=1460$ m/s. Combined with the absence of any observed coherent optical phonon modes, the anomalous hardening of this mode with increasing temperature further supports weak $e$-$ph$ coupling. Together, these results advance the understanding of nonequilibrium carrier and lattice dynamics in kagome materials.