many physics2d & Sprite in same screen occlusion culling questions

hi every one
i try make a game like :

there have so many sprites and with rigidbody2d &Collider2d
with profiler, i know most cpu used in Camera.Render and physics2d.Simulate
for Camera.Render problem
i try to disable can't see Sprite , it simply and work fine for static guard sprite . but move bullet not suit,or maybe i need a optimize algorithm.and what algorithm?
for physics2d.Simulate problem
i try slow down Maximum Allowed Timestep, to limit physics2d compute time. it improve game fps ,but have side effect.
and i try to merger two same type,speed,angle bullet , but algorithm it is complex, i was trying...

does every one has some idea improve performance like this game?
thanks any help~