Why is Postgres on 64-bit CentOS 6 significantly slower than Postgres on 32-bit CentOS 6
Start by using a current PostgreSQL version, preferably a PGDG build. See http://yum.postgresql.org.

Then compare with the same hardware.

There is no known significant performance difference with 64-bit vs 32-bit PostgreSQL on non-Windows platforms.

Personally, I suspect that the 64-bit CentOS might have transparent huge pages enabled, which does cause severe performance regressions with PostgreSQL. See https://dba.stackexchange.com/a/34169/7788, this mailing list post, and Google.

You might also be having issues with the zone reclaim mode for NUMA; Linux's default works very poorly for databases that use shared memory. Set vm.zone_reclaim_mode = 0 in sysctl.conf and apply the changes.