At (so far) three locations the frame rate drops to <1fps. First two are in the monorail intro, when you enter the section with the ICBMs, second is on the elevator just before seeing the helicopter. Third frame drop area is in We’ve Got Hostiles, after going up the elevator to the artillery bombing.
My setup is:
Intel® Core™ i5-2500K CPU @ 3.30GHz × 4
GeForce GTX 760
Ubuntu 16.04, native steam client, playing public beta
Issue persists when all graphics settings set to minimum.