Hi,
You need to write a function or small code which will trace the total execution time for a single method, so you can easily trace that which method is taking too much time. Accordingly you can optimize the code in that method.
MVC has two event i.e OnActionExecuting & OnActionExecuted which cames from System.Web.Mvc.ActionExecutingContext, means these are the methods from interface you have implement these two methods. So in MVC for each action method before exection & after exection you can trace the log.

Slowness is depends upon the which methods you are using calling the data from database like Entity framework, store procedure, ADO.Net connection etc. Also some times in store procedure we use @Table apart from that we need to you #Tables for better performance .
Let me know if you need more detail information.