Tor 0.2.4.23 brings us a big step closer to slowing down the risk from guard rotation. Clients now look at the “usecreatefast” consensus parameter to decide whether to use CREATE_FAST or CREATE cells for the first hop of their circuit. This approach can improve security on connections where Tor’s circuit handshake is stronger than the available TLS connection security levels, but the tradeoff is more computational load on guard relays. Backports several important fixes from the Tor 0.2.5 alpha release series.”]
Source: https://lists.torproject.org/pipermail/tor-announce/2014-July/000093.html