Why is my MVC controller slow when serving images

Question

I have this MVC view that has a list of images, these images are dynamic so they come from a controller. To simplify things this controller has only this code:

    [OutputCache(Duration = 0, NoStore = true, Location = OutputCacheLocation.None)]
    public ActionResult RenderImage(int id)
    {
    return File(@"C:\Users\Pictures\myimage.png", "image/png");
    }

I'm not caching things to prove my point.

My view requests 8 images all with a different id, and the timings are horrible:

Sometimes it's fast for some images (which are all the same) and on each refresh it's different, can't find a pattern in it.

The app is hosted in IIS and the timetaken in the server logs shows it's the server that is eating up the time:

Has anyone a clue why this happens? If I request the images individually (not in the page) it's always fast.

Answer 1

Turns out this is the solution in my case:

[SessionState(System.Web.SessionState.SessionStateBehavior.Disabled)]

user1394965 suggested this as the answer.. but his answer is gone?!