Same for me (cf. 0026962).
This has also been reported in the dev branch discussion:
https://forums.bistudio.com/topic/143930-general-discussion-dev-branch/page-904#entry2949157

When the issue occurs FPS drop from their normal level to 1 to 5 within a few seconds. They then stay on that low level whatever you do (exiting the mission, exiting the editor, restarting the mission...). The only remedy is ending A3 itself. After a restart of A3 it works ok until the issue comes up again after a few minutes of playing.

Still present with todays build.
Maybe something is wrong with memory allocation: In task manager I observed that A3 is producing _lots_ of page faults, though there is still plenty of RAM left.

Adam: Before I encountered this problem I suffered from 0026881.
That issue seems to be resolved by yesterdays build, where I first encountered the FPS drop.

As a software developer I would first check, whether the fix for 0026881 or something in that area may have introduced a regression.

(Unless of course I simply do not encounter the crashes anymore because the FPS drop now kicks in before the crash can happen ;-) )

As far as I can say it seems to happen in every mission I play. I just played with build 133700, and it happened the first time I played after about 10 minutes in the game.
It seems to happen each time you play provided you take your time to finish the mission.

I have no clue what actually triggers the issue. It does not seem to be related directly to certain player interactions or events.

I always play my own simple scenarios (i.e. without much scripting) in the 2D editor.
Geforce GTX 970, 8GB System RAM, Core I7-2600K, Win7 Pro 64bit