I had a 5 member replica set. A secondary node was removed. A new master was very quickly elected. Queries to mongo got extremely slow.

I initialized the ReplSetConnection like so:

::Mongo::ReplSetConnection.new(["ip1", 27017], ["ip2", 27017], ["ip3", 27017], ["ip4", 27017], ["ip5", 27017], :logger => Rails.logger)

When I ran rs.conf() on the mongo command console, the secondary server that was taken down remained in the list.

I was told there must be an odd number of nodes, so I removed the master node and used the instructions at http://www.mongodb.org/display/DOCS/Reconfiguring+when+Members+are+Up to reconfigure the set.

When the two dead nodes were removed, the connection was fast again.