After Rackspace server creation, PostgreSQL query planner doesn't work as expected
I work for Rackspace and would like to help. Can you please post some details on exactly how the image was created? Are these both listed in your control panel as "next-gen servers" or is one "first-gen"? Differences here may explain some of the performance discrepancies you're experiencing.

It would be extremely helpful if you could provide query plans from both servers (http://www.postgresql.org/docs/current/static/performance-tips.html#using-explain). 

A tool like explain.depesz (http://explain.depesz.com/help) may also help you to identify whether the queries are performing the same way on both machines.  

Feel free to post your answers here for the community to help (if you are comfortable) - or drop me a line at shaggy@rackspace.com with details and I'll be happy to coordinate with your support team.