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I have a simple humanoid armature: no IK rigging yet and it worked fine until the arms started messing up when I try to move them. As far as I can tell there is nothing wrong with the weight painting, I even cleared all the weights off of all the vertex groups in the arms but when I repainted them the same thing happened. The mesh doesn't follow the bones at the same speed and deforms wrong and I have no idea why or how to fix it.
Unposed Armature Armature Malfunction

The only modifier I have active is the armature so its not a stack order problem, and none of the other bones have any weight on any part of the arm so I really just have no clue what is wrong here and any help would be greatly appreciated.

EDIT: Here is the .blend file

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  • $\begingroup$ Maybe share the .blend? blend-exchange.giantcowfilms.com $\endgroup$ – scurest Dec 23 '20 at 17:53
  • $\begingroup$ Have you tried Weights -> Normalize All? $\endgroup$ – jackiejake Dec 23 '20 at 21:38
  • $\begingroup$ Normalize All doesn't fix it, sadly, just reduces the strength of most of my weights. $\endgroup$ – ClownCity Dec 23 '20 at 22:55
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Your arm is affected by two extra bones:

Spine.2

Spine.2 weight paint

Spine.1

Spine.1 weight paint

That's why your mesh only follows the arm at "one-third speed"; the other two-thirds of the influence comes from these two Spine bones.

If I quickly clean up the Spine bone's influence, the arm moves correctly.

Fixed arm

Btw, one quick way to check what bones influence a vert is to select the vert in Edit mode and look in the sidebar under the "Item" tab. You can see a list of all groups the vert is assigned to and their weights. See the extra Spine.1 and Spine.2?

Sidebar showing vertex group elements

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