As I've seen from all forum threads, there is no "official" response on the low fps in multiplayer from BI. It is my belief that no one ever reaches a constant fps above 40 and its usually well below that even for people with massive power rigs. I believe it also depends on the map and the mission and the server. For instance the very popular king of the hill servers with 100 max players, everyone is running around in the mid 20's with a max of 30fps and they still somehow find it enjoyable even though at times during heavy player/vehicle activity the game goes frame by frame. Watch twitch.tv sometime and see for yourself.
I've had good mp experiences on wasteland stratis in large player servers that were run by the same people running the koth servers. And I've had good fps in mods such as breaking point on altis. There is no definitive answer probably due to all the variables involved. One thing is for sure, Arma has been and will always be a low fps experience compared to all other multiplayer games despite how ever much money you throw at your system specs.
On a side note, I'd like to see someone at BI make a video on altis with 100 players showing a lot of players and vehicles and explosions going off all with over 40 fps. Guaranteed it will never happen. In fact, I challenge anyone to do this. At this point in time it seems like an impossibility
