Forget whoever told you whater BS about performance.
There’s simply no way that attaching a component costs more than running 2 sets of skeletal meshes. Ever. Particularly on mobile, where skeletal meshes need particular care.
Especially, when (if you read the docs about master pose component) the epic team expressly warns about performance concerns and promotes the mesh merge system.
You have the bones to animate the gun all included in the default skeleton. The weapon IK bones.
They can handle all the transitions and gun tricks you want.
They cannot handle stuff like slide checks, because to do that you need an extra bone for the slide, but normally that’s handled by the gun itself (attach the left hand IK to the slide bone that’s part of the gun and the characters animation will move it along.)
Last but not least.
You are running the gun on the same ABP (with same skeleton and same animations).
Are the gun bones of the AO the same or did you maybe forget to move them around?
If you test the AO on the gun only, does it seem like it moves as it should?
Even a slightly different offset will cause the calculated position to be off (which is why this whole double mesh is a particularly bad idea for weapons).