Most things done with VBA are short enough that execution time is not a problem
unless they are written in a very inefficient way.

Are you doing huge amounts of computation?

Care to post the appropriate parts of your code? There might be some
inefficiencies that can be eliminated easily.

their are several methods that can increase the speed  of the macro....even if
u assign a Null string as VbNullstring
eg MyVar = "" better MyVar=VbNullstring

Wait for few days I'm going to publish a White Paper on this VBA perfomace and
optimization.

If you publish this kind of a web page,it would be very helpful for me.
Because,sometimes i'm making some codes which are not so efficient. Even
ScreenUpdating = False or ActiveWindow.Application.Visible=False is not
equalizing these speed  problems.

If your vba  alters a cell value the whole spreadsheet will recalc. If the
spreadsheet is slow at recalcing this may cause speed  to reduce to a crawls.
One option, if this is the issue, is to turn recalc off  at the start of the
macro and on again at the end. This depends a bit on your algorithm, though.