Yeah I get the exact same thing! That is actually why I came here to The Feedback tracker...
For Example, I was just playing the campaign mission "Moral Fiber" several times this week, & I suddenly get this FPS drop whenever crossroads starts talking about the UKSF...

So I'd say after about 3 mins of playtime, the game nosedives in FPS... -This REALLY should be addressed...

TO add, I am on the Dev.-Branch...

Yeah. Im getting the same problem on devbranch. - I've got no clue whats going on...

-It looks like a recent update to the game seemed to have fixed the problem!

     -But I still gave you the error reports, Hope you have the least amount of inconveniences... 

     -Also, don't worry! Your game (ArmA 3) is amazing and a simple crash here and there is nothing but a minor annoyance. Besides it is what is expected from the devbuild...

     -In addition, I might've screwed up the game a little with various mods, because whenever I actually load into the Virtual Arsenal, The loading screen says "Arsenal - by Karel". Shouldn't it say "Arsenal - by Bohemia Interactive"?

Sorry you had to read so much...
I just wanted to cover everything...

I also experienced this behavior numerous times in singleplayer as-well, but I don't think you can really reproduce this event - the behavior of ArmAs AI is too Sporadic & Inconsistent...

But I have noticed that this typically occurs during very complex missions (Many Units, Waypoints, Scripts, Ect.)...