I don't think you can do much to speed things up. So you may be better off
disconnecting the data source before saving the document, then reconnecting
the source when you need it. You shouldn't lose any of the merge fields in
your document, but you will lose any sort/filter options.

Alternatively, if you always know before you open the document whether or
not you want to use the usual spreadsheet, it might be simpler to keep two
versions of the dcocument, one for use with the "regular" sheet and the
other for when you want to choose another data source.

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Peter Jamieson - Word MVP