I bought Planescape Torment yesterday and installed it.
I tried to run the game with or without mods, with admin rights, with the basic theme, with XP SP3 compatibility mode,...
But the game is still really realllllllllllllyyy slow.
Even videos at the introduction are slow. I don't know what I can do more than that.

Do you have any tips which can help me to play this classic?

My specs:
OS: MS Windows 7 64-bit SP1
CPU: AMD Athlon 64 X2 5000+
RAM: 3.0 Go DDR2 @ 324 MHz
Video card: ATI Radeon HD 4800 Series (Sapphire/PCPartner)

Are you using the DDraw Fix? You can find it here.

have you tried different config options? Turning on software rendering?

Also, maybe something from this thread will help you.

I've tried to unchek Hardware Acceleration with Direct X SDK but it's still slow.
The tricks from the thread didn't help me, sadly.
And it's hard to make tests because introduction videos are so longggg and it's a pain to pass them.

Thanks for the help.

You should be able to skip the videos with ESC key.
In torment.ini, try changing framerate value. Maybe it'll make difference (shouldn't though).

Yes I can but when I press ESC, the screen turns black for 1 or 2 minutes and then pass to the next video...

Play it windowed and have Task Manager brought up next to it.
Does the game eats up all resources? (CPU / memory)

Yes
For the memory, it doesn't seem extravagant but it consumes 50% of the computation of my CPU.

Go to planescape folder and open Planescape ini


Where it says Cache size try putting 1000 instead of the default value.


Alternatively, if it was anything but 1 in default, try and lower it to 1 instead.

Try this:
1. open torment.ini
2. set all video rows from value "1" to "0"
3. set fullscreen mode to value "1" if it isn't already
4. install widescreen patch and set it to your desktop resolution
5. play the game