sql runs fast in ssms slow in asp.net

I have been having this problem for a couple of weeks now. The problem is that the query takes 4-5 minutes to run on the website and at most 2 or 3 seconds to run in ssms. Also I found that after I make a change to this query like adding the customerId variable it will start running quickly on the web page but by the next day it is slow again. The query in question is this one:

Keeping ASP.NET Session Open / Alive

Which is the easiest and most unobstrusive way to keep an ASP.NET session alive as long as the user has the browser window open? Is it timed AJAX calls? I want to prevent the following: sometimes users keep their window open for a long time, then enter stuff, and on submit nothing works anymore because the server side session expired. I don’t want to increase the timeout value for more than 10 min on the server as I want closed sessions (by closing the browser window) to time out fast.