Locking performance issues will often be evident by an excess of clients that are waiting for a lock to be granted. If you join two pg_locks entries together with a matching pair of pg_stat_activity ones, it's possible to find out various information about both the locker process that currently holds the lock, and the locked one stuck waiting for it:
SELECT
locked.pid AS locked_pid,
locker.pid AS locker_pid,
locked_act.usename AS locked_user,
locker_act.usename AS locker_user,
locked.virtualtransaction,
locked.transactionid,
locked.locktype
FROM
pg_locks locked,
pg_locks locker,
pg_stat_activity locked_act,
pg_stat_activity locker_act
WHERE
locker.granted=true AND
locked.granted=false AND
locked.pid=locked_act.procpid ...
