11 spell speed out of context will knock off about .02 seconds from yours/fairy's (re)cast time. That's practically nothing. The extra 3 det? Also pointless. 12 PIE? Close to 100 MP, which also translates to ~+20 Aetherflow. Huge? Not really. I'd argue moreso than knocking off a hundredth of a second and half a point of MND worth of Det, but it doesn't really matter.

That's not to say spell speed for fairy is a stupid idea. You can actually make a spell speed build for just for Eos, summon her, and switch back to your (crit or det) focus build. Since Eos's stats will only be recalculated on a resummon or when using Fey Covenant. But just 1 piece? Pointless.

If you're even considering Fending ring for BiS, I don't know what to tell you.