When multiple builders have towers that are attacking the same monsters at the same time, the server appears to lag out and all the clients will lag for several seconds repeatedly. Selling the towers such that only one person's towers are attacking will remove lag.
Some builders don't seem to cause it, but myself and several other heavily geared ones consistently do. I can take part in games where other people build together and it is ok, but if I add a single tower everyone lags out. So the common workaround is to build everything or nothing.
Some abilities, namely Broomnado and the Ghost Halberds, also seem to cause the lag if other players cast them inside killzones from other player's towers.