Optimization! Or lack of it! Just because the starting area runs at 60fps in your computer doesn't mean the game runs well. There are much more demanding areas later.
For example, there are some places like near the top of the Corvega Plant where I have 60 fps looking to one side and 25 fps looking at the other.
In other words, the framerate tanks when I look at a particular area. Why? Well, that area has a bigger amount of houses than usual, of the type that are not a separate interior level, but are in the general map.
So the game I think the game has huge problems determining where it's what in the Z-plane, to know what to draw on the screen and what to skip. Notice how in the last screenshot most of the screen has a huge polygon drawn (the floor) so most of the houses will be behind it, even then, the framerate is still 30fps.