Hello,
"what actually is the difference between a construction description and a performance specification? if there is one at all?"
There is no difference - some call it construction description (BB), others performance specification, yet others construction/equipment description. All these terms refer to the scope of services purchased under the contract for work signed with the respective provider. For all, the following applies additionally: what is not described at any point in the BB - or at least documented via an additional agreement - is considered not purchased
Kind regards