Multiple CDR sources combine as 1-(1-CDR1)*(1-CDR2)*..., not by simple addition, capped at 75% total - so the fourth or fifth small CDR affix you add is worth much less than the first.
This inverse-multiplicative design is intentional specifically to keep CDR - one of the most powerful stats in the game, since it directly multiplies uptime on cooldown-gated damage/defense skills - from becoming trivially easy to max out.
A 20% CDR source alone leaves cooldowns at 80% of normal length; adding a second, independent 10% CDR source reduces THAT remaining 80% by 10% (i.e. -8 percentage points of the original), leaving cooldowns at 72% of normal (not 70%, which naive addition would suggest).
75% hard cap regardless of source count. Two sources: 20% then 10% CDR -> 1-(0.8*0.9) = 28% total CDR, not 30%.
When choosing between one large CDR roll and several smaller ones of equal total nominal value, the single large source is always mathematically superior once you already own any other CDR.
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