application user and sql login

Question

I have develop a web application in MVC4 it is a application about credit and financing. Application is used by 40 to 50 users at a time. Application reporting is very heavy takes 2 to 3 minutes in generating.
I am using single db user 'sa'
Now I am thinking to create all application users in server 2008 also.
Each application user will connect with own db user not with 'sa' user
my question is that is It right decision?
It will increase my application performance?  If it is not right then how i can increase its performance? as 40 users taking reports with single 'sa' user may be take more time instead off each user taking with its own database user .

Please urgent help is required