Clif,

Thanks for the in-depth help.

I added the code, but I'm not sure there's a difference - although my
design might have something to do with that.

Using Excel 2007, I have an .xlsb workbook that contains two worksheets.

On Sheet1, there are about 1,000,000 cells that each contain sixteen
nested formulas.

1) In each cell, there's an initial IF test that determines whether the
next of three other nested IF formulas should run.

2) Each nested IF formula contains four other function calculations. If
the first IF formula evaluates TRUE, then calculation for that cell stops.
Otherwise, the second IF formula runs and if it evaluates FALSE, the last IF
formula runs. So in total, there's a maximum of about 16,000,000
calculations.

On Sheet2, there's two radio buttons, ON and OFF, that set a TRUE / FALSE
value. That value is what the initial IF formulas on Sheet1 use to determine
if the formulas should further calculate.

So on Sheet2, choosing the ON radio button triggers the formulas on
Sheet1 to calculate. But since Sheet2 has the focus, screen repainting
doesn't occur.

The biggest bottleneck appears to be a third party COM add-in that
provides four custom functions in each of the three nested IF formulas.

Apparently, the COM application that returns the evaluated data takes too
long to do so.

This appears to cause all of the nested IF formulas to evaluate which is
why it takes so long up to 3.5 hours in total.

If there's a way to speed that up, I'd like to know.

Until then, I'll be hoping The Great Pumpkin at Halloween will bring me a
new Intel octo-core hyper-threaded computer.


- Ronald K.