Exactly - if your sim speed is dropping below 0, it's because your CPU isn't meeting demand.  Lowering the settings that hit your CPU is what improves your sim speed.

What solved my sim speed problems, for the most part, was to turn off most of the services in the system configuration.

We had a Single Core 3Ghz system with an Nvidia 9800.
Sim speed was average 4

Then we replaced the motherboard and proc for a Q6600 (4x 2.4Ghz) with the same video card.
Now sim speed doesnt go below 8 on the biggest maps, and is 10 on cataract.

In both Systems memory was 2gb, detail on full.

So looks really like a proc related as yo can see above.

[quote who="[ShakeNBake]" reply="2" id="2312094"]Polynomial, I am running an overclocked Q6600 and a 9800GTX+
^^
Just preordered Windows 7, running on XP at the moment.
My shadows look shit at 1920x1080, all jaggy.
Also I don't think the 9800 likes full HD, not enough memory on the card rly (512MB). Might need to upgrade to one of the newer dual chip cards with 768MB+ memory for full HD.
[/quote]

Strange, i run a 9600GT OC 512mb 650Mhz underclocked @ 500Mhz in fulldetail on 1900x1200 and no problems, 26" display. I underclock to test when games are getting shoppy etc...