Long-time annealing treatments are carried out on Al-Mg-Si alloys in air and in H2 environments and show that hydrogen significantly reduces the precipitate coarsening kinetic. The underlying mechanisms are investigated through classical Oswald ripening theories and simple strength models, which confirms that hydrogen reduces the solute diffusion coefficient. This diffusion delay is also observed during natural ageing of this alloy and Al-Cu alloys. In this system, the hardening response is delayed and it is connected to a slower precipitation kinetic of Guinier-Preston zones. To clarify the mechanisms, ab initio calculations were used to evaluate the diffusion coefficient of the copper in aluminium with and without hydrogen trapped in vacancies at finite temperature.