I've noticed that sometimes I will send an SMS to a friend, on my iMac (10.10.2) on Messages (v8.0 4733), and the "progress bar" at the top will get to about 90% complete, then hang there for up to 3-4 minutes, before finally being "sent." After that first message actually gets sent and I'm given the "sent" sound, the next messages I type / send are sent almost instantaneously.

So, what is holding up that first message and how can I prevent it?

For anybody thinking it's a Wi-Fi or Modem problem, I'm almost always doing something on a various webpages at the same time that I'm doing messages, so that's not the issue.