Well I liked the demo a lot. The secrets were nice, maybe shouldn't talk about some of them because people need to find them. The first one is a bit overwhelming if you don't go to the green door first.
One of the things I think that need more work (for add a different thing to all the stuff the people already write) is when monsters spawn when you do certain objectives or crossing through some areas. If you are looking at the correct moment, you see them appear from nowhere. Example of that is when you grab the SMG on the truck at the beginning. If you grab it backwards you can see the mobs appear where the armor is. Also, you can just skip their respawn if you are enough careful. This happens a lot if you do a normal run through the level.
The jump is unresponsive when you are close to edges, you need to jump way ahead. Is something you can deal with with practice, but not totally sure that is an intended thing. Improve this will be good because, almost for the alpha map, you are rewarded with a better position to deal against certain areas.
The always run option, is something I use. BTW, could be strange to deal with the speed of the running. That is why is handy to change to normal speed pressing the run key (because is hard to be precise with the platforming).
By other hand, I was able to beat the map without take any damage, without attack a single monster, even without grab the sword (Wakazashi) from the start. I got 95 % kills (I think the kills are based on the monsters you found, because if you don't found some, they don't count,) and got a B rank for 11,59 seconds. Here the screen. (sadly didn't found a way to take a screenshot in game)
About it, I'm sure is possible to beat the map under 10 seconds, just need more practice. The end button have a problem, you can go through an be unable to hit it with the action key.