It's the massive increase in the number of clients connecting to the Tor network. It has gone from 500K per day to over 2 million per day. https://metrics.torproject.org/direct-users.png?start=2013-01-01&events=off&end=2013-09-03&country=all At the same time, the advertised bandwidth in the network has actually decreased. https://metrics.torproject.org/bandwidth.png?start=2013-01-01&end=2013-09-03 I've argued before that Tor is mostly CPU-bound, meaning that the crypto operations used to build circuits and push encrypted traffic are the limiting factor on the network. High bandwidth relays max out their CPUs long before they max out their bandwidth. Even with 1 Gbit ports, they can only push about 200-300 Mbit of traffic per core. Some relay operators have posted to the tor-relays mailing list: https://lists.torproject.org/pipermail/tor-relays/2013-August/002594.html So even though the new clients don't seem to be doing much (bandwidth isn't increasing), maintaining open circuits for 4 times as many clients is straining the relays. The whole network is slowed down with increased lag and a high percentage of failures when connecting to sites. Because circuits to hidden services use twice as many relays as circuits to clearnet sites, we should expect hidden service to be affected more by this. As for what's causing it, I don't think anyone knows yet.