Routers running at or near a BandwidthRate setting offer terrible latency and induce connection time-outs. Refuse all DIR-port requests with "HTTP/1.0 503 Directory busy, try again later ".

'tc' ingress filter is dramatically better for rate-limiting.

For case and 'tc' example see

final post

https://lists.torproject.org/pipermail/tor-relays/2015-October/007901.html

initial post

https://lists.torproject.org/pipermail/tor-relays/2015-October/007863.html