When you're seeing crazy-choppy framerate online, it's usually because one or more players has set the game's resolution higher than their PC can support. Their framerate issues become your problem!
First off, try going into your settings and have everyone in your party set VSYNC to OFF.
If this doesn't help, you can try the longer method of troubleshooting:
- Ask all players in the match to lower their resolution to the lowest possible option while in-game
- Play a bit, and make sure the framerate looks good -- if it's still choppy, stop at this step and let me know!
- Pick one player, and have this person raise their resolution
- Play a bit! If the framerate goes back down, this person's your culprit! They'll need to play with a lower resolution than they're currently using. If the framerate still looks good, move on to step 5:
- Repeat steps 3-4 with each individual player until you find the poor fellow whose resolution aspirations are bigger than their system's britches. Castle Crash them into submission until they agree to play with a lower resolution!
You might also want to experiment with different settings to see which provide the best experience for everyone!