Here is an actual example:
a player has a spell speed stat of 435.
Their Physick has a cast time of 1.93 seconds and a recast of 2.41.
After Selene uses Fey Glow, their spell speed stat will be 565.
Physick now has a cast time of 1.83 with a recast time of 2.28.
Even in situations where you don't NEED Eos, you should still be using Eos. Her buffs are far more useful and give you and your group extra "wiggle room". That will give your group insurance against unexpected mistakes and also increase your group's dps far more than Selene ever could.
