If you're concerned with performance and have no interest in portability, mysqli is the best choice.
One of the differences is that with mysqli the prepared queries are implemented on the server side of the database, while with the PDO they are emulated on the client side.
This means that every time you run a PHP script, with the PDO the prepared queries have to be compiled again to generate an unprepared common query, and PHP is spending time recompiling the query and sending the whole query to the query. MySQL server.
With mysqli, PHP only sends the parameters every time the same prepared query is sent to the server when it uses the same persistent connection, even though it is servicing a different HTTP request.
The possible advantage of PDO portability often turns out to be utopian because most applications when they opt for mysql, end up not changing again.