As of now, very much so, I'd imagine. It's why it's still in EA. Optimizations very rarely happen until the end of the development cycle, once it's known not only what, exactly, the game will or will not need to do, but also
how it's best to do it. Any optimizations made early on in a dev cycle are likely to simply need to be undone, overwritten, or handled a completely different way. It would be a rather big waste of time to try to "iron out" the issues while the thing is still being sewn together.
Not 100% sure. I don't see any difference at all performance-wise between the two modes, but I definitely don't try to play either mode in 4K. (Are you sure you're playing in 4K using a 960???

)
As for the maybe why, the single player game is handling a lot of AI, whereas the multiplayer is mostly worried about simply syncing players on the server. So, a battle of several hundred NPCs on each side, each of which needs pathing, damage modeling, individual attack and defense moves, target tracking, morale levels...etc., etc., etc.,...not to mention all of the graphical load...all of which must be handled by a single CPU. That can definitely create more slowdown than the CPU simply managing the packets it's receiving -- pre-processed by the server for the most part. All it's really doing for MP is
displaying the final results. Also, all of the assets for MP modes are pre-cached at the start of the match. No need to load in things it wasn't expecting. That's a lot quicker than trying to account for the thousands upon thousands of asset combinations possible in any SP battle.
Performance in games like this is extremely CPU heavy. There are lots of factors that can affect how "smoothly" a game runs besides just the graphics. I'm guessing the stutter people are seeing will coincide with CPU spikes, not GPU spikes.