Zenity Zadark

My PC is maybe a bit uncommon, which causes some problems to get a decent performance. I have an AMD Athlon X2 5000+ (that's a 2,6 GHz Dualcore) and a Radeon 7750. In other words: An 2006 CPU and a 2012 GPU. Because of this graphics settings just plain doesn't matter – on an almost empty sim I get 22 fps on "low" and 19 on "high"... That won't be a problem – but with more complex sims, things get worse and quite often below 5 fps. So I'm searching ideas to reduce the CPU load (beside draw distance, which I've already reduced).