They could fly faster (most have max speeds >0.9 mach) but are slower to save fuel. The optimal speed would be slightly lower but as airlines and manufacturers

They could fly faster (most have max speeds >0.9 mach) but are slower to save fuel. The optimal speed would be slightly lower but as airlines and manufacturers have settled on 0.85 mach for long-haul, introducing a slower plane would mess up schedules and could be seen as a competitive disadvantage.