Twin Bites flat cooldown reduction is not effective against structures.
The only exception to Dragon FormTwin Bite applying on-hit effects to all affected targets is Titanic Hydra and Ravenous Hydra's passive effect Cleave, as it has an internal cooldown of 0.05 seconds when triggered on different targets. This is a special case to prevent abuse.
Active:Shyvana surrounds herself in flame for 3 seconds, gaining bonus movement speed that decays each second. Shyvana's basic attacks extend the duration of the effect by 1 second (on-hit), up to a maximum duration of 7 seconds.
Initial Movement Speed: 30 / 35 / 40 / 45 / 50%
Decay: 4.5 / 5.25 / 6 / 6.75 / 7.5%
While active, Shyvana deals magic damage every second and 25% of that damage on every hit to enemies surrounding her. Monsters take 20% increase damage.
Dragon's Descent has an effective duration of 20 seconds when not attacking. If attacking continuously with a 2.5 attack speed, it has an indefinite effective duration. With Sated Devourer only attack speed of 1.67 is needed.
Dragon's Descent has an effective cooldown of 150 / 75 / 50 seconds when not attacking. When permanently attacking at 2.5 attack speed, it has an effective cooldown of about 17.65 / 15.8 / 14.3 seconds. Combined with Sated Devourer, the effective cooldown is reduced to 12.24 / 11.32 / 10.5 seconds.