SQL Application SP Performance

I have Application and when trying to login using 5 to 10 users my all SP’s performance is good under 1-2 Sec. but When we enabled 200 users and trying to use same API call from Application and hits number of same SP’s some SP execution is increase to 10 Sec. and some time also Timeout error as well as connection error.
I have couple of transaction tables with 900 millions rows and some hourly jobs running.

What does the SP do. Sounds it is doing more than just authentication.

You might need to simplify that SP to only do auth and nothing but auth

Can I have your email id ? or you can reach me out @ 6pravin9@gmail.com

I prefer to chat here for the benefit of all others in the forum