That will be why then. Interesting.
It's true that my machine is a better spec than yours: i5 2500, 8gb, 1gb Nvidia (720GTX I think).
However, prior to getting hold of that I was using a faulty Dell T7400 with an E4350, 16gb, then 8gb, then 4gb and finally 2gb (can you spot the fault!) with the same GFX card once the OE 512mb Nvidia card burnt out.
With that POS, I mean PC, the game played nearly as well but I could only get about 30-32fps and remain smooth, as opposed to the 45-50fps I get now.
The thing is, the experience got worse as the RAM declined but even with only 2gb I was always able to use all 4 cores and the game was pretty stable. My biggest problem was crashing when I alt-tabbed into another app in Linux (spreadsheet usually) but as a rule, once I was racing things held up well 99.9% of the time.
Looks like 4 or more cores and 4gb or more of Ram might be considered something of a baseline.
But then again, you CAN get GP4 to work well without GPxPatch.... Which means you're using both cores automatically, no? Hmm.... Strange that. I've a feeling we're missing something here. I'll keep thinking about it.
Have you tried a de-tuned f1graphics.cfg Tom? There's one for laptops somewhere which I think I tweaked for use with my T7400 before it died. I know it isn't going to be the answer but I wonder, does it help at all? IIRC it helped quite a bit when I only had 2gb of RAM.
Also, I've ended up running GP4 at 1680x1050 res (in a borderless window) inside my native 1920x1080 Linux desktop. It did this initially because it helped with the alt-tabbing into a spreadsheet issue on the T7400. Not sure it that's relevant to your case or not, but it might be worth a try as I seem to recall it made a big difference for me in terms of overall stability.
I ought to state again though (for the benefit of others who might come across the thread), my current Linux setup as outlined above (i.e. nothing special) runs GP4 with GPxPatch really well and I can use all the mods I want.