Trying to clear contents of a range of contiguous cells. Taking a very long time.

I am trying to clear the contents of a range of contiguous cells (containing formula) in Excel 2010 64bit. But it is taking a LONG time (read >30min to clear a range 288 x 100). Originally was doing it in VBA but it was taking too long so I started stepping through the macro and identified that the slow down was occurring on a particular sheet only. So the problem is in Excel....not VBA because it is still very slow when deleting the same range in Excel without using VBA. I've tried all the usual suspects including: Application.EnableEvents = False (run from the VBA Immediate window when using Excel without VBA), Setting to Example of slow clear contents5.xlsxExample of slow clear contents5.xlsxManual Calculation, Turning off screen updating, deleting all the conditional formatting on the sheet, removing any data validation from the sheet (yes all these at the same time). There are no links to external workbooks. I've even tried in VBA changing from .ClearContents to .Value = vbNullString. All to NO AVAIL! By way of background the file is about 72Mb #.xlsm. If I try to clear contents of larger ranges on other sheets in the same workbook it is instantaneous. If I copy the problem worksheet off to a new workbook, I can delete the problem range almost instantly.I look forward to hearing from anybody who has a solution to why deleting a range of cells should take so long! Again to clarify, I am using the terms "delete" and "clear contents" interchangeably, but they both mean "clear contents" (not delete and move up cells). P.S I've also looked into the reported conflict between Excel and Google Desktop Office Add-in (Office Button–>Excel Options–>Addins–>Com Addins and deselect Google Desktop Office Addin) but I didn't have Google Desktop Office Add-in so it isn't that.