Quick Spell Speed Question

[Neophyte]

Spell Speed and Crit Hit Rate roll downhill; the more you have, the larger the marginal boost from adding more.

Isn't SS the only stat with no dimishing returns?
Afair it's something like ~30 points per -0,1s recast, which is an improvement by -0,4% from 2,5 to 2,49 but an improvement of -0,44% if it's from 2,3 to 2,29.