Due to being unoptimized, you'll need to do the following to prevent slowdown in the future: in the in-game Settings turn down 'Graphical Settings' from HIGHEST to HIGH.

All this does is disable depth-of-field although it's not as significant to the visuals as in other games (in my honest opinion).

Simply press 'SPACEBAR' or 'Esc' to pause the game, select "LOAD" and then click "YES". This will reload the latest checkpoint and the problem will be gone for the time being.

There is no permanent fix for this issue, although it does rarely happen (for me at least, and it seemed random).