Parallel option performing worse on cluster
If you want to run in parallel you have to design for it. Things to consider is using partitions so they can be scanned in parallel. Don't forget that parallelism in general will try to use full scans on segments. Partition pruning can help reducing the size of your scans.

If you really are using pq for this sort of small queries, make sure you have enough PQ slaves ready for use. Otherwise, the starting of new slaves is added as overhead to your query. Not that bad for dss type queries that run for hours but deadly for short running queries.

Also think about using queuing your PQ queries to guarantee that when a query starts, it has all the slaves it needs. This can be done in Oracle Resource Manager.