So I'm having the horrendous FPS in single player. But multiplayer is fine?
MP is totally playable.

Please let me know if you are or are not encountering the same thing.

Yes because the SP has significantly more graphics wizardry and if they did the same for MP no one with a less than a beast of a rig would be able to play. This would result in low population on servers and the sure and instant demise of the MP. This is also one of the biggest reasons COD gmes do so well and still continue to rely on an ancient ID software developed engine.