Is anyone else getting huge slowdown in MDK 1 starting from the "Practice Room"
I have an AMD Athlon 64 X2 and 2X Geforce 7800 (Forceware 175.19), 2GB of Ram, Windows XP

I'd say maybe setting the affinity to one core might help too, AMD X2's have problems with that in some games, right?

I only get slow down when I go into the options menu, it can make it nearly impossible to change some settings. Also get alot of graphical glitches such as textures temporarily disappearing.
Intel Core 2 Duo 6700 2.66Ghz
Nvidia GeForce 8800 GTX 768Mb

Like Tim, I am getting unplayable slowdown when entering the Practice Room. It seems to speed up when I zoom in enough, so it might be a problem trying to deal with the outdoor environment. I have an AMD Athlon X2 (2x2Ghz cores), 2 gig of ram and a GeForce 6800Gs, Windows XP, so pretty similar to him. I will try various options and see if I can get it to work.
Edit:
@ Skreczi: Changing that setting didn't help, thanks though. Any other suggestions? I've now tried all the different options but nothing seems to help.

Thanks, I had the same problem and that fixed it.

Sorry, No luck. Setting it to run on one core didn't help either.

Works pretty well for me, until I turn around, then its slowdown city. I tried the smooth shader setting to no avail.

Same problem with a core 2 6850 and a 8800GT. Solved with "A possible solution to this problem is changing the Smooth Shaded Stream option to OFF
Option - Direct3D - Smooth Shaded Stream"
Thank you for help

I've tried turning off Smooth Shaded Stream but It wont work for me ether, can't even set the core because it locks up when I Ctrl+alt+del out of it.

Having the same sluggish problem here.
Tried modifying all the direct3d settings to no avail.
I can walk into the 'practice area' and it will crawl. As soon as I go back to the previous tunnel/room and the door closes, it's back to normal.
Athlon 64X2 3800
2 GB Ram
Geforce 7600GS
DX 9.0c
Forceware 169.21
XP Service Pack 3

Same problem here plus the "grey radar" issue. Non of the suggested fixes helps.
What allmost cured it, some stuttering remains but way less, was to install the original CD version.

Haven't received any tech help yet, sent in my question a few days ago. Might take a while since they're probably working hard on getting the site working smoothly, but if they get back to me with some suggestions I'll let you know (especially if they work!). The first tantalising section before the slowdown occurs just makes me even more desperate to play it! :)

Me too, I'm wanting to get past the training room but it's unplayable. I only have a single core Amd 64bit 3000+, geforce 6800 vanilla, Win XP SP2, 2GB ram, etc. :( Anyone have a similar setup to any of us? Could be graphic drivers? My driver version is 175.16