Are you playing wesnoth 1.12 because they have significantly improved the rendering performance in the latest version. I know 1.10 could lag my computer but 1.12 is very fast and snappy. Also in my experience, playing 1.12, the game state almost never gets out of sync. If it happens it is because one or more players have different version of some add-ons. So yeah, you really should try 1.12 or even checkout and play the version from their repo because it has even more improvements to it.