which gets large amounts of data, and so takes some time
What it sounds like you are fetching large data and not transferring/sending large data. If so, above config setting would not help as it is for Requests sent. Example, for file upload, you would need to increase request length and timeout.
Since you have not shared much on what you are doing, nor have you shared what you see when you DEBUG, I will suggest you to fetch data in chunks if possible. Assuming you are getting some data to bind with grid, have pagination in it to improve performance here.
If you are keeping your database connection open for too long then it too needs a change. Add timeout in your connection string, and set the commands CommandTimeout property.
For details of the config entry you mentioned in your question, Refer: MSDN: httpRuntime Element (ASP.NET Settings Schema)