[firebird-support] SELECT statement runs at different speeds

[red_october2009]

I have the following statement:

SELECT
  f.PAR_GUID,
  MAX(DATEDIFF(DAY, f.EVT_DT + f.EXP_RSP_DYS, CURRENT_DATE))
FROM
  JET_LTR f
WHERE
  (f.EXP_RSP_DYS > -1) AND
  (f.RSP_RCVD_DT IS NULL) AND
  (DATEDIFF(DAY, f.EVT_DT + f.EXP_RSP_DYS, CURRENT_DATE) >
  f.EXP_RSP_DYS)
GROUP BY 1

The DB is inactive all night. Each morning, when I run this for the first time 
that day, it will take 40 seconds to complete.  However, without any changes to 
the DB, if I run it subsequent times the same day, the subsequent times it will 
complete in less than half a second.

What can I do, to make this statement run in sub-second time, always, including 
the first time it's run?  Also, would using a computed field help the speed at 
all (for the MAX() function).

Thanks in advance.