Yes this game have some serious problems, I have Radeon HD7850 2GB, Phenom II X4 965, 8GB RAM and SSD, the game was runing fine for me but now after the newest patch it's worse, performance deteriorate over time.
Found this on Linux forum version, performance of the game on linux while running in-game benchmark 18 times:
And it seems that the game's performance is decreasing over time (with or without the overlay). After a while you need to restart the game to get back some performance.
Here are the benchmark scores (at auto-detected settings) running for 18 times while monitoring the RAM usage. ( I have 16 GB RAM)