How does cooldown reduction work?

I think it's cooldown "rate" boost rather than hard cooldown reduction.

So, 100% CDR = your cooldown is halved, i.e. you cast twice as often.
200% CDR = your cooldown is about 1/3rd, i.e. you cast thrice as often.

Basically, think of it in terms "casts per second." So let's say throw, at a 10 second cooldown is 0.1 casts per second. At 100% CDR it becomes .2 casts per second, or a 5 second cooldown. At 200% CDR it becomes .3 casts per second, or a 3.33333... second cooldown.

I suspect that the weight based penalty is just additive with the regular cooldown reduction.