CREATE TABLE messages_test (
 id bigint PRIMARY key,
 message text,
 ts timestamptz
);

INSERT INTO messages_test SELECT a.id, a.id::text, now() FROM generate_series(1,5000000) AS a(id);

ANALYZE messages_test;

DO $$
DECLARE 
 v_i int;
 v_exists bool;
BEGIN
 FOR v_i IN 4000000..6000000 LOOP
  v_exists := 1 FROM messages_test WHERE id = v_i;
  IF v_exists THEN
   UPDATE messages_test SET ts = now() WHERE id = v_i;
  ELSE 
   INSERT INTO messages_test(id, message, ts) VALUES (v_i, v_i::text, now());
  END IF;
 END LOOP;
END $$ LANGUAGE plpgsql;
-- 31.377 s

SELECT max(id) FROM messages_test; -- 6000000
SELECT min(ts), max(ts) FROM messages_test; -- 2021-01-26 18:27:47 2021-01-26 18:28:05

-- 24.783 s
DO $$
DECLARE 
 v_i int; 
BEGIN
 FOR v_i IN 4000000..6000000 LOOP    
  INSERT INTO messages_test(id, message, ts) VALUES (v_i, v_i::text, now())
  ON CONFLICT (id) DO UPDATE SET ts = now();
 END LOOP;
END $$ LANGUAGE plpgsql;

SELECT max(id) FROM messages_test; -- 6000000
SELECT min(ts), max(ts) FROM messages_test; -- 2021-01-26 18:29:59 2021-01-26 18:32:10