In addition to overall performance improvements, my version includes a workaround for the horrible FPS in the questbook page (and other parts that display text).
The performance bottlenecks vary depending on the resolution, and apparently also from one graphics card to another; On my computer, at 1680x1050 resolution, the game is much smoother than v1.21
Don't hesitate to report whether you get better or same (or worse?) performance !
Latest download: http://code.google.com/p/arx-fatalis-fixed/downloads/detail?name=arx-fatalis-fixed_v0.03.zip
-Fixed magic symbols not disappearing when closing spellbook
-Fixed NPC detection dots appearing over wrong maps
-Fixed inventory mouse offset
-Increased questbook performance, at the cost of font quality for now
-Fixed main menu back buttons clickable zone to match their appearance
-Increased main menu left side performance
-Increased ingame performance
-Increased main menu performance
-Fixed character speed increasing with computer uptime (also fixes animation problems)
-Ingame minimap shows more of the surrounding map