I can confirm the same experience using a Radeon R9 285 with the Mesa 18.2.* series. The game for the most part runs really smooth but as soon as I reached the bunker shown on James B's Youtube video the framerate dropped greatly and no matter how low I set the graphic settings it made no difference. - Solus OS - Mesa 18.2.1 - R9 285 (Tonga) - Ryzen 5 1600@3.85GHz