create or alter function INT2STR (
    V integer)
returns varchar(11)
AS
begin
  return CAST(V as varchar(11));
end;

CREATE TABLE T (
    ID      INTEGER GENERATED BY DEFAULT AS IDENTITY,
    NAME11  VARCHAR(11), -- = CAST(ID as varchar(11))
    IDSTR   COMPUTED BY (Int2str(ID)),
    IDSTR2  COMPUTED BY (CAST(ID as varchar(11)))
);

-- в Т 100,000 записей

select NAME from T;
-- 639ms

select IDSTR from T;
-- 1s 14ms

select IDSTR2 from T;
-- 718ms