The more options each character class has, the slower combat goes.
Level 10 Fighter with x number of combat maneuvers + standard melee or ranged attacks (multiple per round at this point) + can cast all scrolls + can throw potions + has special maneuvers via weapons
Level 10 Rogue with Sneak Attack + Cunning Action + standard melee or ranged + can cast all scrolls + can throw potions + has special maneuver via weapon
Level 10 Cleric can cast a gazillion spells + melee or ranged + can cast all scrolls including mage scrolls + can throw potions + has special maneuvers via weapons
The higher the levels, the more choices you'll have per class. If you give everyone additional choices, this severely slows the game down. The more choices a player has, the more the player sits there wondering what the heck move they're going to do against the enemies. People complain turn based is slow. Well, yes it is when you have so many options to choose from each round. The more options you have, the slower players take.
One of the main points of limiting choices, especially in the beginning, is to get players used to what their characters can do. You might have one or two special abilities, and over the course of a few levels, you learn those abilities well. Then you gain a few more and learn those also. Now you know which special abilities work best against which enemies as you slowly progress and gain more and more special abilities. Thus, combat doesn't go as slow because you progressively learn what works best against what for your particular class. You become an expert at what each character can do.
