Postmaster only takes 4-5% CPU

Hi all:

When I run a (big) query from pgsql, postmaster used to take almost 100%
of CPU. Now, with aparent no reason, Postmaster only takes 4-5% CPU and
the same query takes really longer to finish.

I have tried renicing -20 postmaster but it still only takes a small
percentage of CPU, while 90% is idle.

This is a "top" screen while running the query:

   2:27pm  up 5 min,  2 users,  load average: 0,80, 0,26, 0,08
74 processes: 72 sleeping, 2 running, 0 zombie, 0 stopped
CPU states:  3,1% user,  2,1% system,  0,0% nice, 94,6% idle
Mem:  1031012K av,  246040K used,  784972K free,       0K shrd,   11496K
buff
Swap: 2040244K av,       0K used, 2040244K free                  196916K
cached

   PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME COMMAND
  1165 postgres   0 -20  3476 3476  2912 D <   4,5  0,3   0:02 postmaster
   268 root      15   0     0    0     0 SW    0,1  0,0   0:00 kjournald

Any idea of how can I get postmaster to eat as much CPU as possible?

Thanks a lot! Ruben.