This edit is cool and all, but you can't even blow up the fire extinguishers.

The only thing I don't like about the original map is the yellow rifts on the ground that are low resolution.

I dig

I attempted to, but the map is pretty much as optimized as its gonna get in this stage.
The reason you're having those issues is due to the dynamic occlusion methods, wherein objects that are completely obstructed by other objects are forcibly unrendered. There's also "frustrum occlusion", which culls objects out that are not currently in your FOV. These sorts of system is normally fine in practice, however they are very CPU intensive when there are MANY objects to render/unrender at once. This can start to cause hitches on some CPUs. If you've ever noticed you hitch more when you're moving fast / turning fast, this would be the explanation. Hellmark in this case, has over 25k StaticMeshes, which is A LOT for one map. The detailing is pretty dense as well. This is especially true for the second area you described.

Have you made any sort of optimizations in this version? One of the problems I had with the original Hellmark Station was that there were occasional stutters as you look/move around.

And then there was that one area with the bar shops that had the cage and the fire underneath; your framerate would drop quite a bit if you walked through there. Same goes for the train station area if you're on the overpass.