Scenario: Login - Click on Add Button - Enter Details - Click on Save Button
10k user
Do I really need 10k login credentials to perform this task or a single login credential can also work?
Does this make any difference in performance metrics?
No you cannot use same credentials to test the multiple user login . However you can check the performance of the app via logging in same user multiple times , but if you need to test for multiple users you need to login with different credentials.
Giving you an example for different users there can be different data for example Facebook , for Facebook user is directed to homescreen where he/she sees different posts , but the feeds screen will not be same for all the user , data will be different.
so to check efficiently you need to login with different credentials to check the multi user functionality.