This is going to sound weird, but having the Stereo-mix disabled is causing the drop in FPS (for me atleast). Is your PC good and should it be able to play this game at High but are you getting insane low FPS? Try this.
I went out of the game, turned it (Stereo mix) off, went back in, 0-3 FPS (I changed NOTHING in the settings). Went back out the game, enabled the Stereo mix (it's when you right-click that speaker icon and go to "Recording devices --> Recording --> Show Disabled devices --> Enable Stereo mix"), again I didn't change anything on the settings and it worked flawlessly on the highest settings with more than 30 FPS.
Now you wonder but why the fuck did you enable the Stereo Mix in the first place, well because I want to play SW: Battlefront II.
If this fix works for anyone else please let me know.
PS. From what I've read from the SW: Battlefront II pages, plugging in (or enabling) a microphone should also do the trick.
EDIT: Well, doesn't look like the above is exactly true, I don't know what's going on with my PC anymore. Turned ot off again and it still continued to work fine with 30 FPS.