I just tried your map, played until I got out of the bookcase area (that was all right?).
I liked the idea of you being in a dream and have to stop it from occurring again. It makes for a good setting to have some otherworldly effects and events such as all those big hanging corpses. That portal was a cool effect too. And it was nice to hear some voice acting.
But there were also loads of not so positive things about the map. Holes in the walls, objects placed through other things, flickering textures (comes from overlapping objects) and decals floating in mid air (I guess you removed an object that had a decal). Objects placed outside the map (you can see the halo of a torch or something outside the map through a wall in one room). Pillars with one side see-through. You used that book you can pull out a bit (from the puzzle in the normal game) like it was an ordinary book. And other things... It honestly makes me wonder if you even tried the map yourself.
The starting room had 10 spot lights overlapping and they were all shadow casting. That's a major slowdown. You could easily remove most of them and no one would notice.
Look at this section of the light tutorial.
Working one room at a time and making sure there are no errors before you move on is a good idea in my opinion. You don't need to bring it up to full detail but at least make the basic stuff (like walls, ceiling, floor and furniture) look good. If you feel you need to plan out the level how about drawing a rough layout on paper before you start working in the Level editor? That's how FG did it
I think that restarting is a good idea because there are too many problems already, but don't give up. It's a cool story idea and I would love to play it.