I'm getting a really horrendous framerate in the underworld parts of the game -- in particular it seems to be connected to having lots of the little torch doodads displayed on the screen.
The particle effects option is already set to low, along with a number of other options that are either set to Low or turned off entirely, but it doesn't really seem to help. It still chugs in those areas, despite running just fine on the overworld areas even with those other settings turned way up.

I am able to run much newer games than this, with much more detailed/intensive particle effects present, so I can't imagine this is an inadequate hardware problem.
Has anyone else had an issue with this?

I have pretty much the same problem though I can tell it's not running good above ground either, at least in the campaign. Normal skirmish with no units right at start I can move camera around fine and seems great, but start a new campaign and it's jerky and once I go underground it's unplayable.

None of the video settings change performance a bit and I even tried emulating windowed mode with d3dwindower to no effect. Tried old video drivers and newest as well with no luck.

Intel E7200@2.54ghz
ATI 5770 1gb
2gb ram
WinXP 32bit.

Obviously nothing special but I run much newer things much better. Only other game I have with inexplicably bad performance is Carmageddon TDR2000, also a pretty old one.

Runs like crap for me as well.

AMD Athlon 2800+ 2.4ghz
ATI HD4650 1gb
1.5gb ram
Win XP 32bit

And there is absolutely no performance increase at all when I lowered and turned off every setting.

I noticed that this only happened when I upgraded my card from an nVidia one to an ATI.

Sorry to bump this but I find it hard to believe that we'd be the only 3 people with this issue, surely someone out there has found a fix?

My fix was to get a new computer. Game works a lot better now.

Given that the new computer (currently) has worse video than the old one (new: integrated Radeon 3000, old: Radeon HD 5450) I'm going to go ahead and guess that the better processor in the new computer is what has helped (new: AMD dual-core 2.8 GHz, old: P4 single core 2.8 GHz).
So, based on my own observations, I'd say getting a better processor, and particularly a processor with more than one core, might help.

That's just bizarre since it seriously ran flawlessly before I got my ATI card.

I've decided to try playing past the tutorial and the problem has extended to the over world too and now I'm getting weird issues like the ground flickering and models flickering.

What the hell.

This surprisingly runs horrible for me. It stutters and plods about, makes it highly un-enjoyable. My rig should eat this for breakfast!

Rig:
8-core Vishera 4.0ghz
8gigs of DDR3-1866
Asus GTX 660 2GB

Same problem here, waiting for solution. :/

My PC - Phenom II X4 975@ 3,9 GHz, GPU - Radeon HD 6850 1GB, 8 GB RAM