Arma 3 has a bit of an odd engine when it comes to network updates. If the server FPS is low or your connection to the server is bad then it impacts your local frame rate. A lot of server admins for their public boxes run multiple servers on the same box and the overall performance is impacted and that destroys the client side FPS for everyione. But if your bandwidth/ping is also problematic you'll see similar problems. Some missions also have very poorly written scripts and these can really drag performance down and ruin the experience. In essence it often isn't something the developers got wrong its a problem with the mission and/or server or your connection to it.
The key to getting a good frame rate in multiplayer is choosing the right servers and the right missions. The A3 server can only run 60 players reliably and then only on a top spec processor. So you should definitely be choosing games smaller than that (we run 40 players despite having a 3570 in that box). Wasteland is especially problematic as are some of the other commonly played missions and you should look for real missions or unique communities instead of the generic missions run everywhere. Most of the generic missions actually don't run very well unfortunately and they don't promote team play, which is largely what makes Arma fun to begin with.
I think a lot of people that play Arma 3 get onto multiplayer and have a bad experience because they play domination/wasteland/evo and never realise these are only there on the public server because they require minimal administration. We are the same at Tier1Ops, we can't always be on the server so its runs our CPG mission when we aren't around. We set a tone of cooperation and leadership however so people who play on our server often organise their games but we prefer to run a real mission when we can.
You have to find that right server and community before you really get the full Arma experience, because its a sandbox of epic proportions so what people build in and around it is what defines how fun the game is. There are so many good mods already, we run 12 or so now along with a load of our own custom scripting and gear. I personally think that is where the best games are, with custom missions gear and scripting that brings together an authentic milsim like experience with a team trained in military tactics. Makes all the difference especially when its on a decent server.