POSSIBILITIES TO BOOST PERFORMANCE
Most important: don't do premature optimization. Always try to find a bottleneck using profiling tools and improve that. Everything else is wild guessing and/or just a waste of time.
Use stored expressions
When working with custom data objects
If you display expressions in a grid-controller and use a custom data object for a column the value is not calculated
with the sql statement. So if the value is needed in the custom data object for instance it's a good idea
$select->expr('EXPRS'); in the
This way it's already calculated with the sql statement and therefore faster when used in your custom data object.