Recently bought a new rig:
Asus P5Q Pro P45
Vista Premium 64bit
ATI HD 4870
Intel q6600 @ 2.4ghz
Im getting around 25 FPS in a skirmish in Supreme Commander. Generally dips to a low of about 10. Having based my choice of system, to a degree on the reviews of this site, (See http://www.tomshardware.co.uk/radeon-hd-4870,review-31046-11.html)
Any one have any idea as to why such low FPS?
My ATI drivers are up to date, as it Vista.
No, nothings wrong with your system or video card, it's your CPU were the bottleneck is. I play Supreme Commander Forged Alliance and the AI spams loads of tech 1 and 2 units. I have a E6400 @ 3.04Ghz and I played against 2 A.I opponents and after about half an hour the game slowed down as the the AI just sent countless tanks and bots to my base.
Each of those units has an A.I tree that it follows and what path is chooses is formulated by some complicated mathematical formulae, give the amount of work the CPU has to do it's no wonder your Computer buckles under the strain.
In future limit the maximum number of units each side can have.