I am having problems with FPS, which is very slow and makes the game quite unpleasant to play. Main menu and everything before the actual in-game content are running very smoothly. I'm guessing somewhat 15-20 FPS. I really need to know if this is normal, since Sacred 2, for example, runs very fine with very smooth FPS.
My specs:
OS: Windows Vista Home Premium SP1
GPU: GeForce GTX 260
CPU: Intel E8400 (Dual-core; 3,6GHz clock)
Mobo: ASUS Striker Extreme
Hard Drive: Seagate Barracuda x 2
Thanks in advance!

It should be running more then fine on a computer like that. I would sugest setting the game to run on a single core incase that is causing some of the lag. Are you playing the game fully zoomed out?

Yes, I am playing the game fully zoomed out, but it would still seem to be pretty choppy while zoomed close or medium range.
I think it's some kind of programming flaw or something. Correct me if I'm wrong.

It could be because its not playing nice with vista, try running it in windows xp compatibility mode. When I played sacred it only lagged when fully zoomed out not while on the middle zoom. I was playing it on a rig much worse then the one you have so it should work.

Running in XP compatibility mode did absolutely nothing. I fear that this problem is unsolveable.

How much Ram do you have and what is your computers page file set at?

I have 4 gigs RAM and the page file shows 6863 M.

Did you try the initial suggestion of running the game on a single processor core? Poor performance is very common problem for games made before the the advent of multi-core processors and the usual fix is to simply run it on a single core.

I apologize for my ignorance, but I don't really know how to run a program using only one core. I tried to search google, but I didn't really find anything.
Can you please tell me how to do that? Thanks.

You can do it one of two ways, either from the Task Manager or using a third party tool. To do it from the Task Manager, launch the game, then hit CTRL+ALT+DELETE to open the Task Manger, find the game's executable on the Processes tab, right-click on it and select "Set Affinity". It is usually best to choose the first processor core (CPU 0).
Using a third party tool to do it is usually easier. Tom's Hardware Guide offers one that I have used in the past and seems to work well:
http://www.tomshardware.com/reviews/bang-dual-processing-buck,815.html

Hey! This was very nice thing to know, but even that didn't help me. Now I am very positive, that this problem cannot be solved. I am very sorry for wasting your precious time.

No offense, but you're giving up way too easily. You've only actually tried two possible solutions and those were just the two most obvious ones. We haven't even looked at other possibilities, like background apps interfering or a driver update for your hardware. Also, this is certainly not a waste of time. Some of us actually like trying to help people. If you want to continue trying, I'm sure Ralackk, myself or any number of other people here could continue to offer some advice.

i'm having the exact same problems as this guy. i have the exact same hardware except my Mobo is a P5QL Pro and different HDDs.
Fraps tells me that i'm getting 30 FPS in the menu. 30 FPS fully zoomed in and 30 FPS zoomed out. it's almost as if it's locked @ 30 FPS.
i've tried setting CPU affinity to Single Core, this did nothing
i've tried lowering the graphical settings and disabling FSAA, this did nothing
i've tried Compatibility mode, this did nothing
i ran it as administrator, and this did help, it upped my Zoomed out FPS to 30, but nothing more than that. an unfortunate side effect of this was the game didn't recognize any of my previous saves. when running as admin, i had to start a new game.
the game is playable, but i wish i could get higher frames out of it. it seems odd i can play oblivion at 1360x 768 and average 53 FPS, but i can't do so in Sacred.
love the site by the way!

Fully zoomed out I run into slowdown in some areas. Even when there are no enemies on screen some places just seem to chug like crazy until zooming in. Rather annoying but thankfully its not too common of an occurrence.