Given that db_select is much slower than db_query, why would I want to use it?
There are 5 reasons to use SelectQuery


You are building dynamic queries with a varying number of conditions, joins, fields and so on. See field_read_fields() for an example.
You want to use so called Extenders. Example extenders are PagerDefault (replaces pager_query()) and TableSort (replaces tablesort_sql()). These allow to add additionally functionality to SelectQuery. See also How do you make sortable tables with a pager with data from a custom table?. An example: node_page_default().
You want to allow other modules to alter your queries. Then you can add so called tags and SelectQuery will automatically call a corresponding alter hook for that tag. I'm am heavily relying on this with my Privatemsg module (We already did that in D6 with a custom query builder).
If you want/need to use the node_access system to only show nodes the user is allowed to see. Just add the 'node_access' tag to your $query. This replaces db_rewrite_sql().
SelectQuery has a few features that help to make your code work the same across all supported databases. For example there is SelectQuery::orderRandom(). And if you have a LIKE condition, ->condition('field', $value, 'LIKE') will make sure that it is always a case insensitive comparison. In D6, you had to use LOWER() for that which was much slower. But AFAIK, there aren't more than this two right now.


If none of these reasons apply for a specific case, use db_query().