The latest official patch v1.4007, titled "Director`s Cut", is available at Moddb.com. Link: S.T.A.L.K.E.R. Lost Alpha v1.4007 DC Torrent!
Install the game outside Program Files.
Max out and turn on all settings, and to improve FPS, set sliders and options as following (ready config):
In the second place:
Disable for 1 FPS gain (from 23 FPS):
Double FPS boost in certain, rather more rare areas with ~>99% GPU usage, and is visible after >1 minute after loading save (e.g. Monolith base. Before and after saves - also settings testing place)