In fact it was a design flaw, though not in the server.

The code for checking whether a username was taken was inefficient, so it was taking a huge amount of time to create a new account. The culprit was this line:

  (some [is dcuser (downcase _)] (keys hpasswords*))

The reason the TechCrunch post slowed the site down so much was not the traffic per se but the number of those visitors who were creating new accounts. I pasted a fix into the repl, so the site should be faster now.

Thanks to Rtm for narrowing the problem down to account creation.