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With relational databases, you absolutely can see subsecond to 30+ second ranges for the same query on the same data, if you don't have proper indexing or stats on your tables.

Indeed! An index can be the difference between a 2-5minute full table scan and a subsecond query.

Depends from your disk system and data set size ... It can also mean 2-5 hours or days versus subsecond query. Try having 6 TB database with integer column. Then select just one row from it. Either it's indexed or not. You don't need to explain that, if query ends up taking forever it wasn't indexed.

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