In https://lists.torproject.org/pipermail/tor-relays/2013-September/002715.html Andy reports some profiling on his fast exit. Top on his list is ``` cycle% image symbol ------ ----- ------ 19.37% tor circuit_unlink_all_from_channel ``` It looks like that function crawls the whole circuit list every time we close a connection. With circuit lists being huge these days, it is plausible that it takes a lot of effort to crawl it.