>The original table, before deletions, took an average of 25 minutes
>to fully backup. The modified table has, so far, taken some 30 hours
>and, extrapolating the current rate forward, looks like it will take
>a further 14-16 hours to complete.

What about your indexes, are each entry close to unique or are there plenty
of duplicates for each entry in the index? I don't know where the slowdown
is supposed to take place, but something will take a lot of time if you
have low selectivity indexes. To avoid this pitfall, add the primary key to
the index, e.g. CREATE INDEX idx_continent ON customer(continent, id)

HTH,
Set