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.

Enable viewcache

Ways to enable viewcache even in dynamic components can be found here.

Use stored expressions

More about stored expressions and how they perform.

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 to add $select->expr('EXPRS'); in the _getSelect()-function. This way it's already calculated with the sql statement and therefore faster when used in your custom data object.