Here is a chart that I made to calculate the effect of various Cooldown Reduction sources on Companion abilities. I hope this resource will be insightful and help with any Companion builds.

It's interesting to note that each source of cooldown reduction is additive, which can practically go as high as 84.6% cooldown reduction, or theoretically up to 89% cooldown reduction. Every Companion skill by default will have 8, 12, 16, or 36 seconds of cooldown. Currently the only Legendary quality Companion item is the Companion's Ascendant Ring, while there are no Normal quality Companion items.

Here is an example screenshot of a Companion with no Quickened gear, no cooldown reduction in their racial passives, and without Telvanni Efficiency:
And here is what those skills look like on a Companion with 12 pieces of Epic Quickened gear, 3% cooldown reduction in their racial passives, and with Telvanni Efficiency active:
Hopefully this shows how important Telvanni Efficiency and Quickened gear are to making Companion skills fire more frequently!