Ya, I think that's pretty normal. Depending on the mission file they're using and the amount of people in the server, performance really gets bogged down. I don't think its something on your end, but on the server side. Alot of mods run great, battle royale is my favorite, but those RP servers or really anything with alot of added content has never run very well.
Edit: this also may help you, it gave me a small boost http://www.gaming-asylum.com/forums/index.php?/topic/2277-arma-3-performance-tweaks-and-settings-guide/