CDR works a bit strangely really...If I remember right each piece of gear reduces the current cd by its listed %, compounding to create a total cdr%.
So the calculation looks like CD=CD*(1-CDR1)(1-CDR2)(1-CDR3) and so on.
What this means is that a single CDR rating of X % allows for 1/(1-X%) more uses of a skill, scaling asymptotically up to 100% which would give 0 cd and therefore infinite use (But no cdr is approaches that high, its usually 50% at most, and thats for single skills as a legendary affix, 10% is the highest general CDR rating)
However when compounding CDR ratings, each rating allows successively more uses creating an exponential scale, two separate 25% CDR boosts will reduce the CD to ~56% of what it was, allowing for 77% more use of the skill. This is consistent with math as each 25% CDR boost gives you 33% more use of the skill from the previous value, and 1.33^2 gives ~1.77.
So what the hell does this mean...
CDR scales differently when you are increasing a single rating (upgrading gear for instance) versus stacking different ratings across different pieces. These differences in scaling also make a smaller amount of higher CDR ratings more effective than a large amount of small CDR ratings (Using the former numbers as an example. 2 25% CDR boosts give 77% more uses, while a single 50% will give 100% more). However it still effectively stacks with itself, growing exponentially more powerful the more that is gathered.
The biggest reason people's dps begins to suffer after stacking so much CDR is because they have to give up other damage affixes that compound with so many other factors that they are more effective at their current levels than what the new CDR would give.
So the calculation looks like CD=CD*(1-CDR1)(1-CDR2)(1-CDR3) and so on.
What this means is that a single CDR rating of X % allows for 1/(1-X%) more uses of a skill, scaling asymptotically up to 100% which would give 0 cd and therefore infinite use (But no cdr is approaches that high, its usually 50% at most, and thats for single skills as a legendary affix, 10% is the highest general CDR rating)
However when compounding CDR ratings, each rating allows successively more uses creating an exponential scale, two separate 25% CDR boosts will reduce the CD to ~56% of what it was, allowing for 77% more use of the skill. This is consistent with math as each 25% CDR boost gives you 33% more use of the skill from the previous value, and 1.33^2 gives ~1.77.
So what the hell does this mean...
CDR scales differently when you are increasing a single rating (upgrading gear for instance) versus stacking different ratings across different pieces. These differences in scaling also make a smaller amount of higher CDR ratings more effective than a large amount of small CDR ratings (Using the former numbers as an example. 2 25% CDR boosts give 77% more uses, while a single 50% will give 100% more). However it still effectively stacks with itself, growing exponentially more powerful the more that is gathered.
The biggest reason people's dps begins to suffer after stacking so much CDR is because they have to give up other damage affixes that compound with so many other factors that they are more effective at their current levels than what the new CDR would give.