Well, OP did have WP-Cache installed once; not sure if he reinstalled it after the migration.

WP-Cache was in place, but HN hammered the server anyway. Typically it is not an issue but "dat traffic spike" was more or less way too much for the poor linode to handle:

http://i.imgur.com/3jPD0iB.png

I think Wordpress + Apache is just a beast - I've seen people talk about running Ngnx in front of Apache as a proxy serving only static content but I haven't gotten around to investigating that kind of setup yet.