The developers of MySQL make big leaps forward with each release, and regularly address the issue of performance. MySQL 4 brought with it the new query cache, which stores the results of each of your SELECT queries in RAM. If the same query comes in again and the table hasn't changed, MySQL will simply load the result from RAM and return that - much, much quicker than having to perform the whole query from scratch.
If you do a lot of SELECT queries, and have queries that return the same results all the time or even some of the time, the query cache will raise your performance substantially.
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