Tuners frequently measure the time taken by a particular query. If this time is high,
many tuners will try to reduce it. Such effort, however, will not pay off if the query
is executed only seldom. For example, speeding up a query that takes up 1% of
the running time by a factor of two will speed up the system by at most 0.5%. That
said, if the query is critical somehow, then it may still be worthwhile. Thus,
localizing the problem to one query and fixing that one should be the first thing to
try. But make sure it is the important query.