Performance issues during backup

Hi

We perform nighty base backup of our production PostgreSQL instance. We have a script that basically puts the instance into back mode and then backs up (tar) the /Data directory and then takes it out of backup mode.
Ie,
psql -c "SELECT pg_start_backup('${DATE}');"
tar -cvf - ${DATA_DIR} --exclude ${DATA_DIR}/pg_log | split -d -b $TAR_SPLIT_SIZE - ${BACKUP_DIR}/${BACKUP_NAME}
psql -c "SELECT pg_stop_backup();"

The size of our database is about 250GB and it usually takes about 1 hour to backup.
During this time, we have performance issue where queries can take up to 15secs to return where normally it takes 2 to 3 seconds.
During this time (1:30am) usage is low (less than 10 users) on the system.

Has anyone experience the same problem and any suggestions where to look at to resolve the problem?

Thanks.
Dylan