:LoadAnimation() from the humanoid class is deprecated and could have issues. Try loading the animation from the humanoid’s Animator instance and see if that fixes the issue.
When you use :LoadAnimation(), it returns an AnimationTrack which you can then play.
You are creating two different animation tracks. It’s not playing the same one that you adjusted the speed for.
Do this:
humanoid = script.Parent.Humanoid
local animationTrack = humanoid:LoadAnimation(script.Animation)
animationTrack:AdjustSpeed(1.5)
animationTrack:Play()