What are the docs required for Performance Testing?

The main document is always SRS.Performance testing is also same as other here we have to see to how many user's our application can feed up request at one time, how minimum resources it required to work, how it behave when it is under stress.

Performance Testing tests the systems ability to meet performance standards, such as maximum 3 secs response to a users request.

means for 'secs'

secs means seconds

BRS
SRS

The docs required for performance testing are as follows :
BRS, SRS, Use Case Doc and the Benchmark Doc

The answer to this question depends on the industry, company, and/or project.  In my industry and company, the performance criteria are furnished in the application's specification document.  However, I have also seen this information disclosed in the business requirements document. 

Business requirements doc and functional requirements doc.

Business requirements details the high-level requirements from business standpoint. So some of these would eventually become transactions during performance testing. BRD will sometimes have performance related requirements, in those cases it becomes THE most important document.

From functional requirements document, QA can form different test scenario that can also be used as transactions in performance testing.

NFR: Non Functional Requirement is perquisite doc for PT. It define SLA, entry and exit criteria of PT.