Too fast would be something like a high damage item, in this case lets say it's 100 damage, that breaks after 2-3 shots. Sure it does high damage, and could basically kill anything in one hit, but it wears down so fast it's basically unusable unless you carry a massive army's worth of repair kits with you. No one is going to take the time to use said weapon because its decay rate is so crazy it's unsustainable, even for people who do focus on repair.
On the other hand, a weapon that does only 20 damage, yet doesn't break until after 1000 shots, is a weapon that decays too slow, as it allows you to do many more times damage then a higher end item, which it shouldn't, without breaking, making it the objectively better gun, which it should be an objectively worse gun from having lower damage.
Then we have to factor in things like conidtionals, such as a gun could do 80 damage, lower then the "best" gun which does 100, but, it has armor ingoring capabilites that allow it to do more, overall damage, becuase more of it's getting through the armor then the 100 damage gun, so its decay rate should be slightly faster, based on how much more damage it does on avarage, then the 100 dam weapons which doesnt ignore armor.
Then we have to factor in things like perks, how the armor system works to negate damage, what sort of, if any, penatlies lower item condition inflics beyond simply lower damage/armor rating, etc. etc., and how those effect overall avarage damage and armor rating.
It's honestly such a complicated mess that its no suprise almost no one actually does item decay in games anymore.