lag

Which region do you have your Multiplayer Portal set to and what is the Ping for you at that region?

For example, if you're on the East Coast of the United States and you have your region set to North America- East then you should have a low ping and not see any latency at all, but if you're in Australia and have your Multiplayer set to North America- East then you could be seeing a ping value way up around 250+ and since you really want a low ping with an upper threshold of maybe 130 a value way above that is going to experience an insane amount of latency and packet loss which would make trying to play the game impossibly frustrating.

If you're doing Co-Op, Garden Ops or Graveyard Ops, you're at the mercy of the Internet Connection speed and region of whoever is hosting the game. Connect with someone with less-than-ideal internet speed and your 4 person Co-Op could noticeably go to garbage when that third or fourth player joins and if the host is in another part of the world your connection to them could suffer from high latency which would make it tough on you but not necessarily them.