Most of the time with our Operations there is a lot of stuff loaded in like the enemy, roadblocks, bases etc.. on top of that you have 55 to 65 of us with aircraft and others vics so the frames always tend to be a lot lower than you would have on say the campaign or something like that.
Generally for the ops i would just recommend not running at Ultra, yes your PC might be able to run it but with all the extra stuff on top it certainly wont help your frames at least from my pov. Hope this helps however I'm sure more technical PC wizards might give you more in depth explanations.