In short, whenever you add a limb to a object, the entire object seems to get reconstructed or manipulated in some way. I came to this conclusion after noticing how slow the add limb command gets the more and more limbs you add. I was always under the impression that the purpose of adding limbs in sequential order was to prevent just this, so only the NEW limb mesh would be added.
Check the source and see for yourself. Bug or not this needs to be fixed. Objects have too many limitations, we cant afford to have equal problems with limbs as well.
All you need is zeal