Does CDR increase in effectiveness as you buy more of it?

Just a random thought.

Let's say you had an ability with a 10 second cooldown. You buy 10% CDR, and now the CD is 9 seconds. So effectively, for every 9 times you could cast the spell before (assuming you cast it the moment it was up) you can now cast it 10 times. Doesn't feel very substantial, as you're only eliminating 10% of the cooldown.

Let's jump right to 40% CDR; you'd be casting a spell every 6 seconds. Compare that to 30% CDR, where you'd be casting a spell every 7 seconds. 7:6 is a slightly better ratio than 10:9. By buying 10% additional CDR when you're already at 30%, you've reduced the current CD of that spell by around 14%. So buying that 10% CDR at 30% has made a bigger difference than buying it at 0%.

To illustrate my point further, let's take a few less realistic examples (without a CD cap). Let's say you're sitting at 80% CDR, so you can cast the spell every 2 seconds. Buy 10% CDR, and you'd cast it every 1 second. So you've decreased the effective CD by 50% (2 seconds to 1 second) and doubled the rate of casting said spell. Buy another 10% CDR, and you'd have decreased the effective CD by 100% (1 second to 0 seconds), giving you a casting rate that's only limited by how fast you can click.

Point being, is it really worth it to only buy 10% or 20% CDR and leave it at that? Or should I always try to achieve more CDR, even if it means sacrificing one or two tank/damage items?