HI @Bodhisatya Ghosh
As you mentioned there is no issue in publishing the application, you are facing issue in performance of the application and its slow. So you need to follow the best practices as mentioned above by all. I think should first start with Service studio -> True Change, there are some performance warnings need to work on those first.
After that you can check for service center logs for slow queries, slow server actions and screen actions. By this way you will be able to identify that which part is performing slow and can work on that particular method/action to rectify the logic according to best practices.
Regards.