On the iron and adminspotting side I would recommend some of this and such reading. Hints: key_buffer size in server config file, optimize table maintainance, memory resources, etc.
On the coding side, I don't really think using arrayref over hashref would be much of an optimization, and this kind of tricks. Still, using select * from table is kindof a php thing (sorry, i couldn't resist). :)
Proper indexes and LIMIT statements will prolly speed things if it is really too late for database proper redesign.

... imho :)
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