Salesforce Performance testing

Hi,

I am working on salesforce UI’s and now same code is deployed in Production where I am expecting a heavy load.

How can I ensure that whatever UI’s created are able to handle a load.

When generated a load of 20 users , I saw white pages,  for CSR (50) user at the same time , UI got slow ie, UI is painted in a long time.

Please suggest tools , process so that 500 user load / performance testing ?

Thank you in advance.

If want to discuss in details please let me know we have a detailed discussion.