An updated version of this post can be found here: Updating a cape physics system to be used for flags
Maybe you could try weldConstraints instead of welds.
The model already uses weld constraints.
Have you thought of rigging it instead of using ball sockets?
Can you elaborate, this is not the kind of thing I typically do.
You can use a rigging plugin to rig your flag, which lets you directly play animations into the flag.
The reason I am using ball sockets is because the flag needs to have realistic motions. What I mean by that is if you were carrying a flag in real life and turned left the flag would slowly sway to match your movement, and this concept works with the current flag, just not when you apply it as a tool on the player.
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I’ve had this problem before, and my fix was by making all the parts inside the too Massless (there’s a boolvalue in the properties of each part where you can find that). However, I’m not sure if that would affect the swaying emotion you want to have.
They already are massless 
I think it has to do with constraints having collisions or smthg idk.
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Yea it has something to do with the ball constraints, as when I disable them it does not jitter.
I think the problem is that the limits are too small. I completely removed them and it works fine, just the flag would glitch out and “snap”.
btw increasing the restitution also seemed to help a bit.
That does a little bit to help but it still does not solve the issue.
I believe that the RodConstraint is one of the problems.
When disabling it, I wasn’t tossed around as much, and after 1-2, I could move. However, there’s still some resistance, which is leading me to believe there are multiple issues causing this.