There are some PL/SQL performance features that are only available in compiled database code. For example, the result cache:

declare

  l int;
  
  function f ( p int ) 
    return int result_cache as
    retval int;
  begin
    return retval;
  end f;
  
begin
  l := f ( 1 );
end;
/

ORA-06550: line 5, column 12:
PLS-00999: implementation restriction (may be temporary) RESULT_CACHE is disallowed on subprograms in anonymous blocks

Shifting PL/SQL as-is from APEX -> database packages may have little or no direct performance benefit. But it does open up tuning possibilities not available if all the code is in your APEX app.