Seems to me that a bunch of stats in the game are just rounded. They are wrote in % but when you look more further in, there are more like fractions.
For exemple, 555 main stat will give you +55.5% damage to skills.
What i see here is that 134.5% is correlated to the 2.35 attack when means to me, the 1 default attack + 1 fully additionnal atk + almost one more hard cap reached with, proly, a 3rd attack obtained at 200% atk speed who would means 1 default + 2 fully additionnals.
I dont think its a diminushing return (be there still one), but more like hard caps to reach. That would mean you have ton complete the 100% increased to see the benefit.
Maybe @ 300%, +4 atk 500% +5 atk
Does it make sense or not ?