Hydride superconductors under extreme pressure exhibit record-high critical temperatures as a result of their exceptionally strong electron-phonon coupling (EPC). To clarify how such strong EPC influences nonequilibrium carrier dynamics, ultrafast hot-electron relaxation in calcium hydrides ($\mathrm{Ca}{\mathrm{H}}_n, n=2$, 4, 6) was investigated using momentum-resolved first-principles quantum dynamics. A two-stage relaxation process was identified: an initial momentum relaxation, where phonons randomize electron momentum with little energy loss, followed by an energy relaxation dominated by optical phonon emission that transfers electronic energy into the lattice. With increasing EPC from semiconducting $P{6}_3/mmc-{\mathrm{CaH}}_2$ to superconducting $Im\overline{3}m-{\mathrm{CaH}}_6$, both processes were found to be markedly accelerated. In the superconducting phase, the energy relaxation was completed within several tens of femtoseconds, demonstrating ultrafast energy transfer from electrons to the lattice. A fundamental regularity is thus revealed: while stronger EPC enhances $T_{\mathrm{c}}$, it also governs an ultrafast channel for initial carrier thermalization and energy injection into the lattice following a perturbation. This defines a distinct early-stage microscopic pathway for energy redistribution in strongly coupled superconductors.

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• Electron relaxation
• Electron-phonon coupling
• Hydrides
• Nonequilibrium systems
• Superconductors
• First-principles calculations
• Molecular dynamics