Tor sometimes waits much longer than ShutdownWaitLength
An operator reports their relay hangs for more than 10 minutes when sent a SIGINT: https://lists.torproject.org/pipermail/tor-relays/2017-March/012149.html
They are also seeing the legacy/trac#20423 (moved) issue: https://trac.torproject.org/projects/tor/ticket/20423#comment:34
Perhaps the fix for legacy/trac#20423 (moved) caused this? Perhaps they have a bad clock?