Apparently, call_user_func() is slow. References:

• php.net
• fab2s/call_user_func
• Optimize perf by replacing call_user_func with dynamic variables symfony/symfony#29309

We're using a couple (functions-l10n.php and functions-plugins.php as of writing) so the idea would be to replace them:

// before : 
call_user_func( $function1 );
call_user_func( $function2, $param );
// after :
$function1();
$function2( $param );

While at it, an is_callable() check before calling would be nice, to ensure the function is actually callable when it's user defined (plugins) (and make sure the cases of calling something callable and not callable are covered in a unit test)