Did you continue a game with the optomization? or start a new game?
Either way, it is starting to sound like a lag issue.
Cause that behavior is often around when at least player has a high ping...
You using wireless?
Lan, or ICO?
You know how to ping your friends comp? how long is it taking? more importantly, any lag spikes or lost packets?
They are using vanilla, so they are missing some of the streamlining that entrench and diplo brought (along with the extra burdens of more stuff to keep track of...) an the upcoming patch is diplo only... hmm...
As a last ditch thing, you could try hardwiring both comps to the same router. From there, go one step at a time back to your previous set up, and see where the weak link is in your connections.