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Ok, I found the problem thanks to the loan of drMerry's database. It is caused when you have a very large number of duplicates in the group. The problem isn't the search algorithms themselves but the code afterwards which repartitions the groups taking into account exemptions. The performance of this is diabolical in the situation of having say 70 duplicate titles in the group, so something badly wrong in there. Will post a new version when I figure out the exact cause and fix it or dump the approach in favour of another.
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