the games frame-rate drops quite a lot in my experience even with just a few grunts in open environments. please tell me i am not the only one experiencing this

Here we go.

I haven't played far enough to experience this.  Which level?

the first one 

i even saved the video if anyone wants to see

It happens  in the opening cut scene. I saw it once a couple of minutes into gameplay. Other then that it's rock solid so far, and a total non issue.

with me it started happening when it started raining 

Its because it wants to make it seem even more epic.

so by it being skippy  and slow it is epic?

I experienced this too during the part where you first come into contact with the Covenant in the first level as well as the first cutscene. After the first level though framerates have been solid with very minor exceptions of scenes with loads of enemies and explosions on screen at once.

oddly this doesn't happen in firefight with 50 enemies