user_previous_login_problem__tuned.sql

....

SET FROM_DATE = $RUN_DATE-7;
SET TO_DATE = $RUN_DATE;

/* We don't need much from before $FROM_DATE, just the max(login_date) */
CREATE TEMP TABLE temp_previous_login__pre_window AS
  SELECT username       AS username
      , MAX(login_date) AS pre_window_login_date
  FROM sales_site.login
  WHERE login_date < $FROM_DATE
  GROUP BY 1;


CREATE TEMP TABLE temp_previous_login AS
WITH base as (
  SELECT *
      , LAG(login_date) OVER (PARTITION BY username ORDER BY date) AS previous_login_date__in_window
  FROM sales_site.login
  WHERE login_date between $FROM_DATE and $TO_DATE
)
SELECT cur.*
      , coalesce(previous_login_date__in_window, -- null means it happened before $START_DATE, or never at all
                 pre_window_login_date)          as previous_login_date
FROM temp_previous_login curr
LEFT JOIN sales_site.login prev
    on curr.username = prev.username
;

....