Need a Java Update?
I think I read somewhere that Lion isn't shipping with the newest version of Java. Try updating the plugin? Otherwise I'm not sure. Did you download the client, or are you trying to run it in a browser? That might make a difference. It may be a coincidence, too--if you're using Multiplayer and they upgraded when you did, the remote server might be the source of the lag. For both 1.7.2 and 1.7.3 the pistons slowed everything down for me.