With raw SQL, the code that interacts with the database will run faster as there are less abstraction layers to go through. However, since those abstraction layers are built in compiled C, rather than PHP, the actual difference in execution time is negligible. What you will be losing is the simple way the ORM allows you to run queries without hand coding each query.
I'm a big proponent of efficiency, but I haven't noticed any slowdown by using ORM/PHQL, and my development time is much, much lower because I can do stuff like this:
$Applicants = $Program->getApplicants()
Instead of
$Applicants = [];
$program_id = 3;
$query = "SELECT * FROM `applicants` WHERE program_id = $program_id";
$result = mysql_execute($mysqli_connection,$query);
while($row = mysqli_fetch_assoc($result)){
$Applicant = new Applicant();
$Applicant->program_id = $program_id;
$Applicant->name = $row['name'];
// and so on for every attribute