Is this expected to slow stuff down on db1047 – or are the queries I'm running just horribly inefficient? ;)
What Dario said is all accurate.  But to answer your question directly, yes, db1047 should be a little slow right now.  Maybe a lot slow.  Because Sean is copying all the data from it to db1048.  This is expected to finish within single-digit hours from a couple hours ago, so it should be fine soon.