I want to paint the stark contrast in experience between two paladins, one played casually, on played fairly optimized.

Where are we? Facing Ketheric Thorm, first fight, at the top of Moonrise Towers

Tav paladin ability scores and relevant equipment:

1. Casual player: Paladin level 8, wielding The Blood of Lathander, Str 20, Cha 16, no relevant feat or offensive equipment*
2. Optimizing player: Paladin level 9, wielding The Blood of Lathander, Str 18, Cha 16, savage attacker, Killer's sweetheart, whispering promise, duelling gloves

* Plenty of usually good equipment makes little difference in this particular fight - KT often fights alone and has "magical plate 2" that takes away many sources of small damage bonuses.

I'm going to make a bunch of assumptions. One of them is that the casual player doesn't use "examine" on enemies all that much. And so they don't learn that Ketheric is undead - at least not right away. The casual player also missed a few plotlines, so they're not level 9 yet. That's a significant difference, but relevant to the discussion, I think. Many casual players of BG3 are a little under-leveled (or worse).

To hit (vs AC 22, excluding advantage):

1. Casual: 50% [ 5 Str 3 profiency, 3 BoL = 11]
2. Optimizer: 75% [ 4 Str 4 profiency, 3 BoL, 1 elemental weapon, 2 gloves, D4 bless = 15-18]

Elemental weapon is available from level 9. The optimizer knows that KT has great AC and chooses gloves accordingly. We here assume that the casual player hasn't burnt all their elevel 2 smites in the previous part of the assault. The optimizer also knows to save one or two level 3 slots. Potions could be relevant here, but I chose to ignore them.

Damage - optimizer has a stored critical:

1. Casual: Weapon 12 (D6 Str 5 3 BoL) Smite2 18 (4D8) - 4 = 26 [normal]
2. Optimiser: Weapon 21 (2D6 Str 4 3 BoL 1 EW) EW 6 (2D4) Smite3 60 (10D8) - 6 = 81 [critical]

So we see a situation in which the casual player has 50% chance to do 26 damage, while the optimizing player has 75% chance to do 81 damage. This illustrates the vast difference in how two players can experience the same fight.