You don't need to performance test Salesforce. In fact, it wouldn't go very well for you if you tried to stress test salesforce. Salesforce will terminate your tests if they see you pulling too many resources. It is a cloud based platform and they don't allow any customer of any size the privilege of jeopardizing other customers performance for their own sake.
To ease your mind:
Do you need to stress test the Force.com platform?
No. The Force.com platform serves over 1 billion transactions daily
with an average response time of under 300 milliseconds, and
salesforce.com tests and monitors the platform to ensure that it has
You should definitely take advantage of the platform’s efficiency–but
not by writing inefficient code. Salesforce.com guards against highly
inefficient code by using governor limits, which help ensure both that
your users have a consistent application experience and that resources
are available to all salesforce.com customers. (One limit, the Apex
Apex script statement limit, is being phased out in the Winter ’14
release, but its original reason for existing was the same.)
Your goal should be to use load testing instead of stress testing. If
you try to stress test, you’re likely to reach governor limits or have
salesforce.com terminate your tests so they don’t affect shared
resources on the multitenant Force.com platform. Performance test your
anticipated loads instead.