I guess that the optimal way to reach upper X from the bottom X would be going without the need of using any diagonal.
My best call to that intuition and feeling against going diagonal is when we are in a PvP situation. A classical thing to combo an enemy (curiously, it is very hard to discover who used this tactic first) was the "paralyze diagonal surfing" (I am adapting what Brazilians used to call that in Portuguese). Going diagonal is sensibly slower compared to straight walking. So if you are paralyzed and receives a diagonal push (an enemy pushes you and makes you go diagonally to somewhere), the enemy team can combo at this window opened by the diagonal movement + paralyze. And even you use a turn to heal, a second paralyze can come right after the first... All at once, that can be deadly. It is a way to for example, combo and kill an enemy with a lot of hitpoints (for example, a high level knight).
So, although I never walked a lot using diagonal, I would go with that on situational conditions only. The regular movement is pretty much all that you need. For PvP and PvE, diagonal is most definitely something that I don't recommend on playing for too long due it being a somewhat slower movement. Be alert to go diagonal if you are about to be trapped (and face a full box of mobs hitting you, for example).