Usort very slow

I have a prolem similar to this post : PHP sort array by number in filename

I used this usort() solution:

usort($photos, function ($a, $b) {
   return strstr($a, '_') <=> strstr($b, '_');
});

Array is like this:

array (
  0 => 'photo-Cname_0111.jpg',
  1 => 'photo-Aname_0112.jpg',
  2 => 'photo-Bname_0222.jpg',
  3 => 'photo-Cname_0222.jpg',
  4 => 'photo-Aname_0333.jpg',
  5 => 'photo-Bname_0333.jpg',
)

Problem is when the array has 1000 elements it takes about 9secs to complete.

How can i make this sort faster? Thank you