Is it because of polygon count or animation, or something else?

Also, whatever the case... do small/hinge rotator with disableBaseMesh set to "True" cost more than basic JointRotator?
Thanks (for good answers, hopefully!) in advance:)
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To save you some time, here's the performance cost of each part as listed in the game:
-JointRotator-1 (Rotator, ie basic) = 7.1
-SmallRotator-1 = 14.3
-HingeRotator-1 = 19.5