Quick Question:
Why tf would anyone ever take the UNCOMMON Frostbolt Relic? It is a net dps drop on its own, and it lowers the attack speed of a unit that is focused on slowing the opponent.
Even under the most charitable situations, assuming that the attack speed (instead of halving), is increased in such a fashion that it is halfway to double the original CD, that still wouldn't make sense for a tower focused on slowing enemies.
The only reasonable way I can see the CD justification, is if it becomes AOE.