I am amazed how slow after effects are and think that maybe something is with my pc.
I have intel core2duo@3,00ghz, 4gb ram, sata hdd, blackmagick decklink studio i/o card and geforce 8600gt video card. It is so slow I cant even play uncompressed SD movie in realtime... If I put keylight on I get 10fps. I also use edius 4 and its render is super faster than ae's... I can put many effects and it still plays in real time. So is this normal?
Non-Linear Editors and After Effects handle video in completely different ways.
With your setup, it seems quite normal, yes.
AE is not intended for real-time playback. You render a portion of the animation/composite out and watch that.
You really shouldn't press your spacebar to preview (I'm not sure why that's even included as an option) Press 0 on your number pad to initiate a RAM preview, you will see green start showing up on the top of your timeline, these are the rendered frames. Since you don't have a lot of RAM, I wouldn't expect it to give you a terribly long preview, but that depends on the resolution of your comp. It will either hit the limit of available RAM and start playing back or you can press 0 (on the number pad) again to play back real-time.
[Robert pukas]"...I also use edius 4 and its render is super faster than ae's... "
Well, After Effects isn't anything LIKE your editing application.
It's an effects, motion graphics and compositing application. It's intended to be used for totally different things than an editing application. Thus, it doesn't work the same way your editing application does.