Application Pool Queue in the IIS has the default value of 1000. Application pool queueing takes place whenever IIS is unable to dequeue enough requests to keep up with the rate of incoming requests.

If enoough requests are Queued, additional requests will be rejected by 503 “Service Unavailable” errors.

You can increase the default from 1000 to 5000 using following steps.

To increase the value for the Queue Length property for Classic ASP:

  1. Click Start, point to All Programs, click Administrative Tools, and then click Internet Information Services (IIS) Manager.
  2. In the Connections pane, select the Web server, click to select Features View, and then double-click the ASP feature. ( MIND it you need to Select ASP feature under WEB server not Web Site )
  3. Click to expand Limits Properties under Behavior, click Queue Length, enter the desired value for Queue Length and then click Apply in the Actions pane.

This is a server level property so modification to this property will affect all websites in your Server.

To increase the value for the Queue Length property for .Net Application:

  1. Click Start, point to All Programs, click Administrative Tools, and then click Internet Information Services (IIS) Manager.
  2. In the Connections pane, select the Web server, click to select Features View, and then double-click the ASP feature. ( MIND it you need to Select ASP feature under WEB server not Web Site )
  3. Click to expand Limits Properties under Behavior, click Queue Length, enter the desired value for Queue Length and then click Apply in the Actions pane.

This is a server level property so modification to this property will affect all websites in your Server.

  1. Click Start, point to All Programs, click Administrative Tools, and then click Internet Information Services (IIS) Manager.
  2. In the Connections pane, select the Application Pools, It will list all associated Application Pools in Right Pane.
  3. Right click the Associate Website Application Pool and select Advance Settings
  4. Under General, Click Queue Length, enter the desired value for Queue Length and Click OK in the Advance Settings pane. 5000 is recommended value

This is a Website level property so modification will only affect the all website under that application pool

Related Article

https://docs.microsoft.com/en-us/dotnet/api/microsoft.web.administration.applicationpool.queuelength?view=iis-dotnet