For you, optimal tyre on normal road is probably a light tyre around 25mm at around 100 PSI

Heavy 38mm tyres will always be slower on the road than this optimal config (more rolling losses, more weight, more aero penalty). But indeed 40 PSI might no be the optimal for these 38mm, and you can work on the pressure to find at least the optimal level to lower rolling losses for these 38mm tyres. It will never be as efficient as an optimal config, but could be better than your current config.