The problem:
While playing the Hangar 6 R&D campaign, I experienced occasional lags during battle.
Basically, every 10 seconds or so, the game would freeze for around 2 seconds.
This lag doesn't occur in the main campaign, nor does it occur in the prep room.

The fix:
The only way I found to fix this, is to disable PhysX.
In my case, I disabled both particles and cloth, but it's possible the lag is only caused by one of the two.

Note:
If you read Suspended aircraft room crash fix, you may have attempted to replace the PhysXCore.dll in order to enjoy PhysX despite the crash. Unfortunately, you'll still have to disable PhysX for Hangar 6 R&D, unless you're fine with the game freezing all the time.

Hopefully, this'll help you guys enjoy Hangar 6 R&D as much as I did (after fixing the lag, that is)