Does that 5% even matter?
5% CDR is a lot more powerful than it sounds due to how %reductions work, and how %reductions are very different from a %increase.
The equivalent of casting spells 100% faster is 50% CDR
The equivalent of casting spells 200% faster is 66.7% CDR
The equivalent of casting spells 300% faster is 75% CDR
The equivalent of casting spells 400% faster is 80% CDR (URF)
The equivalent of casting spells 500% faster is 83% CDR
You need less and less CDR to hit each stage, where with things like AD, AS, HP, Armor, MR, HP/5, MP/5, Mana, etc. it's the same flat rate for each jump. If it takes +100 to go from double to triple, it takes +100 to go from triple to quadruple, etc.
In this case of CDR it's only +50%, +16.7%, +8.3%, +5%, +3% to do the same thing.
While the CDR cap is obviously 40% or 45%, my main point is that each point of CDR becomes more and more powerful the more you already have.
The jump from 40% CDR to 45% CDR works out to be a pretty solid ~9% boost in the rate at which anyone can cast spells, which is a pretty substantial boost to get from just 5 mastery points.