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 Tiamat 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, dealing magic damage to nearby enemies each second, increased by 20% against monsters, and gaining bonus movement speed which decays by 15% of its original value each second.
「 Magic Damage Per Second: 20 / 32 / 45 / 57 / 70 (+ 20% bonus AD) 」「 Magic Damage Per Second Against Monsters: 24 / 38.4 / 54 / 68.8 / 84 (+ 24% bonus AD) 」
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.