How many are too many?

Firstly, you might have better luck if you convert the footnotes to endnotes. If that doesn't improve the speed then you may need to show that information a different way and delete the footnotes.

If you are working with an old computer or one lacking in RAM then it might be worth trying the file on a newer machine to see if that improves the responsiveness.

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Andrew Lockton
Chrysalis Design, Melbourne Australia