Amazon RDS: PostgreSQL Performance

Question

We use postgresql as the backend for Apache. We plan to migrate our PostgreSQL from an EC2 instance (m1.xlarge + 500GB volume / Ubuntu) to RDS. However, in our testing on an instance of the same size (m1.xlarge) in RDS, the performance is much slower (perhaps a third of the EC2 instance).

Both instances are in the same availability zone as the Apache server. Given that an RDS instance will cost almost twice as much as an EC2 instance, what steps or configuration changes should I try to improve the performance of the RDS instance? There are some tips for creating a VPC that needs to be removed from both the web server and the RDS instance. Does the VPC idea have legs?

Thanks in advance.