iMessage loads very terribly and often doesn't work

When I click iMessage, the first thing it does it takes about five seconds for anything to come up. Sometimes my messages come up, sometimes the "new message" screen that you get the first time you ever open it does.

If I do get my messages to appear, it's another five second wait after I click on someone's name. It then usually only shows the most recent message. After opening and closing a few times, it appears as normal, but then takes a few minutes to send a message.

After one is sent, it typically works without a hitch, but only for that person. The process restarts if I message someone else.

This problem began when I upgraded to 8.3 last week.