Site columns pages (mngfield.aspx) loads very slow (more than 3 minutes)

Hi,

Site columns page of subsites on my production farm loads very long. It takes not less than 3 minutes to load it. It is happening for all subsites, whereas Site columns page of the site collection works well.

Also it is impossible to open Site Columns of any subsites using SPD (I am getting: "An error occurred while trying to fetch data from your SharePoint site").

The following PowerShell cmdlet for my subsite runs very fast:

$web.AvailableFields.SchemaXml | Out-File c:\temp\fields.txt

In the ULS I can see a lot of stings (more than 3000) like:

[Forced due to logging gap, cached @ 05/29/2013 08:20:43.03, Original Level: Verbose] SQL connection time: 0.0594349281822131
[Forced due to logging gap, Original Level: VerboseEx] SQL IO Statistics: Procedure {0}, {1}
[Forced due to logging gap, cached @ 05/29/2013 08:20:45.05, Original Level: VerboseEx] SPRequestParameters: AppPrincipal={0}, UserName={1}, UserKye={2}, RoleCount={3}, Roles={4}
[Forced due to logging gap, Original Level: VerboseEx] Reverting to process identity

And right before loading completes I can see that some resources were not disposed during the request:

Leaving Monitored Scope (SharePointForm Control Render). Execution Time=168222,944536247
An SPRequest object was not disposed before the end of this thread.  To avoid wasting system resources, dispose of this object or its parent (such as an SPSite or SPWeb) as soon as you are done using it.  This object will now be disposed.  Allocation Id: {1695290C-4861-4C41-80F5-DC2BDFFF88C9}  
This SPRequest was allocated at    at Microsoft.SharePoint.Library.SPRequest..ctor()     at 
Microsoft.SharePoint.SPGlobal.CreateSPRequestAndSetIdentity(SPSite site, String name, Boolean bNotGlobalAdminCode, String strUrl, Boolean bNotAddToContext, Byte[] UserToken, SPAppPrincipalToken appPrincipalToken, String userName, Boolean bIgnoreTokenTimeout, Boolean bAsAnonymous)     at 
Microsoft.SharePoint.SPWeb.InitializeSPRequest()     at 
Microsoft.SharePoint.SPWeb.EnsureSPRequest()     at 
Microsoft.SharePoint.SPWeb.InitWebPublic()     at 
Microsoft.SharePoint.SPWeb.get_LanguageCulture()     at 
Microsoft.SharePoint.SPWeb.get_Title()     at 
Microsoft.SharePoint.ApplicationPages.FieldListRenderer.Render(HtmlTextWriter output)     at
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)     at 
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)     at 
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)     at 
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)     at 
Microsoft.SharePoint.WebControls.AjaxDelta.RenderChildren(HtmlTextWriter output)     at 
System.Web.UI.WebControls.WebControl.RenderContents(HtmlTextWriter writer)     at 
System.Web.UI.WebControls.WebControl.Render(HtmlTextWriter writer)     at 
Microsoft.SharePoint.WebControls.AjaxDelta.Render(HtmlTextWriter writer)     at 
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)     at 
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)     at 
System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer)     at 
System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer)     at 
Microsoft.SharePoint.WebControls.SharePointForm.Render(HtmlTextWriter output)     at 
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)     at 
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)     at 
System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer)     at 
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)     at 
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)     at 
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)     at 
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)     at 
Microsoft.SharePoint.WebControls.UnsecuredLayoutsPageBase.RenderChildren(HtmlTextWriter writer)     at 
System.Web.UI.Page.Render(HtmlTextWriter writer)     at 
Microsoft.SharePoint.WebControls.DeltaPage.RenderToBase(HtmlTextWriter writer)     at Microsoft.SharePoint.WebControls.DeltaPage.Render(HtmlTextWriter writer)     at 
Microsoft.SharePoint.WebControls.UnsecuredLayoutsPageBase.Render(HtmlTextWriter writer)     at 
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)     at 
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)     at 
System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)     at 
System.Web.UI.Page.ProcessRequest()     at 
System.Web.UI.Page.ProcessRequest(HttpContext context)     at 
System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()     at 
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)     at 
System.Web.HttpApplication.PipelineStepManager.ResumeSteps(Exception error)     at System.Web.HttpApplication.BeginProcessRequestNotification(HttpContext context, AsyncCallback cb)     at 
System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context)     at 
System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)     at 
System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)     at 
System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus)     at 
System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus)     at 
System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)     at 
System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)

I have tried a lot of different things, but can't make it work for about a week already.

Does anybody have any ideas what can be cause of this issue or what can I try for troubleshooting?

Every suggestion is very appreciated as I ran out of ideas.

UPDATE1: Even subsites which have no site columns misbehave. Also, it seems that every another call to Site Columns page takes longer and longer.

Regards Michael