How to speed up audit trigger?
You could alter the trigger with brute force INSERTs as follows:

CREATE TRIGGER trigger1 
    AFTER UPDATE 
    ON tbl1 
    FOR EACH ROW 
BEGIN 
  DECLARE insert_style INT;

  -- I'm interested in two fields only and if they were changed only 

  SET insert_style = 0;
  IF new.y != old.y THEN 
      SET insert_style = 1;
  END IF;
  IF new.country != old.country THEN 
      SET insert_style = insert_style + 2;
  END IF; 

  CASE insert_style
      WHEN 1 THEN
          insert into z_lot values
          ('','tbl1',new.id,'y',old.y,new.y);
      WHEN 2 THEN
          insert into z_lot values
          ('','tbl1',new.id,'country',old.country,new.country);
      WHEN 3 THEN
          insert into z_lot values
          ('','tbl1',new.id,'y',old.y,new.y),
          ('','tbl1',new.id,'country',old.country,new.country);
  END CASE;      

END