update command question?

i have the log analysis report by using the pgFouine. in the part of
Slowest queries, the update commands " update com_status set
travel=620.70001220703 where no=1"  have the high Av.duration(s). how
should i do to solve this problem? why this simple command can take
up
the most time?

Av.duration(s) :21.06
Times executed: 9
total duration : 3m9s