I have played for two hours now, trauma mod, and only completed the first level.

GREAT. Played less than waiting for the game to load levels though, that's the reason, and my computer is pretty decent so the problem is from the game but don't worry if you're in the same case, it's a common issue.

Managed to find some tips around the web to reduce these ♥♥♥♥ however, here's what to do, it's quite simple.

Create a folder called "maps" in your "Painkiller: Overdose/Data" folder. Located on your "Steam/Steamapps/Common" folder, as usual.

An empty folder, just named "maps", don't ask me why it's not here from the installation files, but it will be used for caching files and textures.

Bad optimisation you said ? Here's why...

The most important for me.

Okay, so i'm not the kind of people who likes to play with "omg so powerful computer have 900 fps on Duke Nukem 3D, great gaming experience" has it is useless with most games except a few ones (Quake III anyone ?), it tears the screen and makes your GPU running hard for simply nothing, but it had fixes the overlong load times for me. Once again, don't ask me why, it's plain stupid but it worked for me and others.

Disable the V-sync for Overdose from your videocard settings (Nvidia panel / ATI Catalyst center), apply, then launch the game, it should be way faster now.

Disabling V-sync makes your computer running the game without any restrictions. It results in very high FPS, absolutely useless above your max refresh rate for this game, so your GPU will basically run faster for nothing. Setting max FPS limit after disabling V-Sync will not reduce the improved loading times.

Open up the console ingame ( ², under the "escape" key), and type " /setmaxfps XX ".

XX supposed to be your refresh rate, 60 for most people, more with recent monitors.

These three small steps reduced the loading time for me from more than two minutes to less than 30 seconds, quite an improvement for a game meant to be hardcore, fast and without slowdowns in its gameplay...

Still unbelievable that moderns rigs can't hold this game properly, i didn't played it when it was released but I don't even want to imagine how ♥♥♥♥ed off people were.

Hope this served for you, feel free to ask questions / suggestions.