create index on communities.slug
create index on communities.slug

The test query I tried on the production DB got about 30 times faster.

mccolgst/www.gittip.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,gratipay/gratipay.com,eXcomm/gratipay.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,gratipay/gratipay.com,studio666/gratipay.com,mccolgst/www.gittip.com,studio666/gratipay.com,eXcomm/gratipay.com,studio666/gratipay.com,gratipay/gratipay.com,studio666/gratipay.com,gratipay/gratipay.com