Your best bet for consistent behaviour across those setups (HTML5, iPhone in power-save mode etc.) is to uncheck vsync and leave frame cap at 0. This way the engine will measure elapsed time every frame instead of trying to query the hardware for monitor refresh rate.
Unfortunately this may result in (maybe) less stable physics simulation since the timestep will not be constant. The work with decoupling physics timestep from engine frame time would resolve this, but it’s not planned for any release in the near future.