I wrote a database speed test script a few weeks ago, that measured the elapsed time for running a query 5 times and iterating over the result set each time. The result set in my case contained somewhere around 13.000 rows.

I ran this three times and averaged I got these results:
- Direct Oracle: 3.30 seconds
- Direct PDO: 1.20 seconds
- ADOdb + Oracle: 5.32 seconds
- Zend_DB + Oracle: 4.11 seconds
- Zend_DB + PDO: 1.83 seconds

So I'm going with PDO.