LXDE is a window manager...

I recommend you learn using Debian first, if you are not already familiar.

Nope, Easy Debian can't slow your N900 down, I mean, there is no reason. It's just like a program which runs, when you want it to. You can close it... Also, when I have it open and I'm not doing anything, it uses 3 or 4 percent of my CPU.

And yes, you can run Debian applications if you install it... The ones which are for ARM architecture of course.