Depending on the ORM (or otherwise) it may have some level of caching, so the best performance would be using the PHP functions/classes directly (eg: PDO/MongoC

Depending on the ORM (or otherwise) it may have some level of caching, so the best performance would be using the PHP functions/classes directly (eg: PDO/MongoCollection/Redis). In addition to the above, PHP extensions would be extremely preferable to pure-PHP libraries for database interaction simply because the extensions will be written in C and lack the overhead of the language.