There isn't anything you can do about that other then getting a faster internet connection. When you change targets on the first player the message is sent to the server and then the server will send an update down to your client letting you know what/who they have targeted. However long that takes is how long you have to wait after changing targets before you can assist or focustarget or party1target or whatever you do.

I have simply taken up targeting the next creep before the first is dead (not too early, just a second or so).