PHP Database Functions vs. PEAR::DB vs. ADOdb (and PDO): Joseph Scott does some testing to remind us that while database abstraction layers are nice, there is a performance price to be paid — sometimes an expensive one.
If you need performance over supporting multiple databases and use of PEAR packages that require PEAR::DB (like PEAR::DB_DataObject) then stick with calling the database functions directly. No way to beat that from a performance perspective.
Joseph, how about trying the ADODB PHP extension? According to them:
[It] provides up to 100% speedup by replacing parts of ADOdb with C code. ADOdb will auto-detect if this extension is installed and use it automatically.