slow server : s_lock and _bt_checkkeys on perf top

Hello All,

Master db size 1.5 TB
All postgres 9.1.13 installed from RHEL package.
It has streaming replica and slony replica to another servers.

Server performance is slower than usual, before that, there's a big query
got cancelled and then performance get slow.

No sign of IO wait.

on sar, it's %user and %system dominate the cpu usage
01:25:04 PM CPU %user %nice %system %iowait %steal
%idle
Average: all 51.91 0.00 12.03 0.66 0.00
35.39

on perf top, i saw
18.93% postgres [.] s_lock
10.72% postgres [.] _bt_checkkeys
almost always at top.

I don't have any idea, what's causing it or how to resolve it ?

Any answer is very appreciated.

--
Regards,

Soni Maula Harriz