what deffrentce beetween serverpeer and ClientSDK for performance test.

I'm developing load test.

and looking at performance test.
https://doc.photonengine.com/zh-cn/onpremise/current/performance/performance-tests

It's says
"But if you try to build a "load test client" with Photon's Client SDKs, the clients will be the bottleneck and you will get bad, misleading results."

now I'm wonduling what deffrentce beetween serverpeer and ClientSDK.
because I want to make a Real Performance test (I will use couple of server for client)

could you tell me advantage or disadvantage or any limitation as well.