I created an iCal event at a quater to 5pm EST today (Monday, September 9th, 2013).
It arrived in my inbox shortly after 7:30pm EST today, or put another way, it took almost 3 hours for the calendar invite to arrive.
That is bad.
My general practice (for just this reason) is to include myself on the invite list, so I know when the invite arrives. With iCal, I notice there is frequently a delay, usually about 5 minutes, when I send out an invite, before it is recieved. Sometimes it can be longer. Today took the cake. Almost three hours !!!