c# - service bus receive not returning immediately -

i'm using azure service bus topic/subscriptions , running unexplained performance receiving messages.

according msdn:

if 0 passed in serverwaittime, there no wait time. instead server bus fetch whatever messages available, or return null result.

in code, stripped down bare essentials, pass in 0, receive takes 60 seconds complete. when completes, passed null object. why take 60 seconds when should return immediately?

var client = servicebusclient.getorcreatesubscriptionclient(topicname, subscriptionname, false);  var msg = client.receive(timespan.fromseconds(0));