There are some issues coming both before and after migrations with very rapid fire robot indexing you may say, some of it is a bit rogue and not yahoo or google type indexing. This makes a sudden huge load on the server when 100’s of intensive pages accessed at the same time.
This isn’t the tehcnial detail as I don’t even know all that, just know I’ve been there when it hits and gove from 0.3 loads to extreme in mere moments.