pg_basebackup blocking all queries with horrible performance

Greetings,
I have a 4 server postgresql-9.1.3 cluster (one master doing streaming
replication to 3 hot standby servers).  All of them are running
Fedora-16-x86_64.

http://wiki.postgresql.org/wiki/Lock_Monitoring

I'm finding that I cannot runpg_basebackup at all, or it slows down all
SQL queries from running until pg_basebackup has completed (and the
load on the box just takes off to over 30.00).  By "blocks" I mean
that any query that is submitted just hangs and does not return for
seconds or sometimes even minutes
until pg_basebackup has stopped.   I'm assuming that this isn't
expected behavior, so I'm rather confused on what is going on.  The
command that I'm issuing is:
pg_basebackup -v -D /mnt/backups/backups/tmp0 -x -Ft -U postgres

Can someone provide some guidance on how to debug this? Or is there
some way to reduce the performance/priority of pg_basebackup so that
it has much less impact on overall performance?

thanks!