This is just some information about things we found in our environment.

If you had UltraWebGrid1.DisplayLayout.UseFixedHeaders = True 

and used

        UltraWebGrid1.Columns(iCol).Header.Caption = "Customer"
        UltraWebGrid1.Columns(iCol).Header.Fixed = True
        UltraWebGrid1.Columns(iCol).Width = Unit.Percentage(8)

in the  UltraWebGrid1_InitializeLayout Sub

Rendering the page in IE8 compatibility mode would take approx 4 minutes to for a 10 column, 626 row grid. With compatibility mode off the page displayed immediately.

By commenting out or setting UseFixedHeaders to False the grid rendered immediately in both modes.

It seemed like DisplayLayout.UseFixedHeaders = True and Header.Fixed = True together caused the issue.

Within the ASPX code we had to make the following changes:

-  Within DisplayLayout.GroupByBox

   -  Replace the GroupByBox.Style with GroupByBox.BoxStyle

-  Within DisplayLayout.Pager

   -  Replace the Pager.Style with Pager.PagerStyle  

-  Within DisplayLayout.AddNewBox

   -  Replace the AddNewBox.Style with AddNewBox.ButtonStyle

-  If you are using the GroupByBox

   -  Replace the GroupByBox.Style with GroupByBox.BoxStyle

-  If you are using the DisplayLayout.Pager.Style

   -  Replace the Pager.Style with Pager.PagerStyle

-  If the grid is in a container, the grid width should specify less than 100%.

   If 100% is used the last column will be truncated.

   -  Replace:

      UltraWebGrid1.Width = Unit.Percentage(100)

      with

      UltraWebGrid1.Width = Unit.Percentage(98)

This seemed strange but fixed the issue. As I stretched the browser window I could see more of the last column. This made me believe it was some kind of rounding issue on the individual fields in relation to the whole.

For the Excel Exporter we changed the following 2 lines of code:

Dim workbook As Infragistics.Excel.Workbook = New Infragistics.Excel.Workbook()

to

Dim workbook As New Infragistics.Documents.Excel.Workbook()

workbook.ActiveWorksheet = workbook.Worksheets(0)

to

workbook.WindowOptions.SelectedWorksheet = workbook.Worksheets(0)

Everything seems to be working well using IE8 in both modes.

good luck.....