I think the detail level made a pretty big difference. I definitely ran it in low and kind of forgot that high was an option, but the shotgun animation in that video is bringing up some traumatic memories.
I had a 386sx@25MHz too and I don't remember it being that slow. Unless that demo has the detail cranked up to high or something like that. Although, like that first commenter I had a math co-processor, so maybe that helped.
Or maybe my memory is off and I made the window tiny.