In mongoid.yml if you have the following:

hosts:

• myhost1.mydomain.com:27017
• myhost2.mydomain.com:27017
• myhost3.mydomain.com:27017

And one of the hosts goes down (e.g. kill the myhost1 server) then the entire application becomes slow, presumably because mongoid is pinging a host which doesn't exist. Removing the myhost1 line fixes the issue.

This issue has bit us in production twice and it's very annoying because the application doesn't die entirely, it just becomes slow. While I'd like to get onto Mongoid 5.x as soon as possible, this issue is severe enough that I believe it merits a backport fix.