I don't think it's rounded. In this case, the ability will go from a 4-second cooldown to a 2.4-second cooldown. I think it's easy to underestimate the impact of cooldown reduction on low-cooldown abilities, so consider it this way: Imagine a teamfight or skirmish that lasts 12 seconds (which is close enough to typical). If that ability is used on-cooldown, with 0% CDR it could be used 3 times during the fight, whereas with 40% CDR it could be used 5 times. For a level-5 Transfusion from Vladimir, that's 460 + 120% AP damage and 110 + 50% AP healing. Put differently, if we consider Vladimir's Q damage output (ignore the healing) during the fight, the damage from 0 AP and 40% CDR is the same as from 383 AP and 0% CDR. (Deathcap + Zhonya's would be 312 AP for comparison.) So, CDR is valuable on short-cooldown abilities because it lets you use them multiple times in one encounter, whereas it's valuable on long-cooldown abilities because it makes it more likely that you'll have them up for the next encounter. Honestly, CDR is good on all abilities (unless an ability somehow had almost no cooldown, which would probably be broken), but I imagine the theoretical "power trough" of CDR would be on medium cooldown abilties (like 50 seconds maybe?) that are unlikely to be up twice in one fight but very likely to be up by the next fight.