An ergonomic chair assists you in experiencing reduced physical discomfort with fewer distractions because it helps your comfort last. That alone results in your improved focus and reduced mental fatigue.
Your brain keeps getting interrupted by way of physical cues when your seat is not comfortable: a sore back, a numb leg, or even a cramping shoulder. This kind of discomfort pulls at your attention from your work and makes tasks feel longer and more exhausting.
Your body must be well supported within. When that is the situation, then those distractions do fade from view. Focus will be yours without shifts stretches or adjustments to yourself always. You use energy better because sustained comfort is useful. Another advantage is the feeling of being less mentally drained at the day's end.
Yes therefore, to switch to an ergonomic chair could make a noticeable difference within how tired you might feel physically and mentally also after what would be a long day of work.