When I played this game I noticed the loading times are super long, even when you're just reloading a checkpoint or quicksave in a level. Painkiller Black Edition doesn't have this issue. Thus I'd like to share this guide I found on Steam: https://steamcommunity.com/sharedfiles/filedetails/?id=285147649 credits to the creator for finding it.

For me, just the first step fixes it, basically just create a folder called "maps" in your "Painkiller: Overdose/Data" folder that will be automatically used for caching. Maybe the later steps are needed for faster loads, but it's already enough for me.