Follow up from:
We need to do the following, some of this might need to be split into sub-issues itself:
1. Do some micro-benchmarks of db_query() vs. db_select() to try to get solid numbers on how much slower db_select() is in terms of building a query.
2. Identify the performance cost of initiating the db_select() class in the first place - does autoloading select.inc and query.inc cost anything vs. just including them? Does including them at all make a difference, both with and without APC?