drop procedure if exists test_performance_issues;
create procedure test_performance_issues(Method integer) returning varchar(32) as ExecTime;
  define b      bigint;
  define d      double precision;
  define x,v    integer;
  define rc     varchar(32);
  define s      set (integer not null);

  begin
    on exception in (-206)
      create temp table tmp_test_main_data (
        RecId integer primary key
      ) with no log;
      update statistics low for table tmp_test_main_data;
    end exception with resume;
    delete from tmp_test_main_data;
  end;

  for x=1 to 5000
    insert into tmp_test_main_data(RecId) values(x);
  end for;

  if Method==1 then
    let x=1;
    foreach select m.RecId
            into   v
            from tmp_test_main_data m
      insert into table(s) values(v);
      if x==4000 then exit foreach; end if;
      let x=x+1;
    end foreach;
    let b=GetHiResTimer_Start();
    select count(*) into v from tmp_test_main_data m where m.RecId NOT in s;
    let d=GetHiResTimer_Finish(b);
  else
    begin 
    on exception in (-206)
      create temp table tmp_test_intersection (
        RecId integer
      ) with no log;
      update statistics low for table tmp_test_intersection;
      end exception with resume;
      delete from tmp_test_intersection;
    end;
    let x=1;
    foreach select m.RecId
            into   v
            from tmp_test_main_data m
      insert into tmp_test_intersection(RecId) values(v);
      if x==4000 then exit foreach; end if;
      let x=x+1;
    end foreach;
    let b=GetHiResTimer_Start();
    select count(*) into v from tmp_test_Main_Data m where m.RecId NOT in (select i.RecId from tmp_test_intersection i);
    let d=GetHiResTimer_Finish(b);
  end if;
  let d=GetHiResTimer_Finish(b);
  let rc=(d/1000)::numeric(6,3)||'s, '||v||' recs';

  return rc;
end procedure;

update statistics for procedure test_performance_issues;

execute procedure test_performance_issues(1);
execute procedure test_performance_issues(2);

drop procedure test_performance_issues;

Successfully connected to tb as user informix.
Procedure successfully dropped.
Procedure successfully created.
Statistics successfully updated.
exectime
--------------------------------
1.575s, 1000 recs
1 rows selected.
exectime
--------------------------------
0.031s, 1000 recs
1 rows selected.
Procedure successfully dropped.

select count(*) into v from tmp_test_main_data m where m.RecId NOT in s;

select count(*) into v from tmp_test_main_data m where m.RecId NOT in (select * from table(s));

select count(*) into v from tmp_test_main_data m where m.RecId NOT in (select * from table(s));

select count(*) into v from tmp_test_Main_Data m where m.RecId NOT in (select i.RecId from tmp_test_intersection i);